~ Important Authors Note (Pls Read) ~
Ok so... I've had this oneshot on the back burner mainly because I didn't really know what to do. However, I finally got an idea and decided to do it. So basically this oneshot is based off a discord comment by Syzygy/Pinklollipop from the SolRuin discord that's about Evil Ruin liking Solar and how he'd act around Solar.

However, I am changing up a few things. One of which is adding Yandere to that comment. The other thing is that it's  based on the SAMS episode "Hatsune Miku is in love with Solar" only it's changed to have Ruin around with Solar and Moon in the daycare's gift shop as well. So as you can expect based on the title of this oneshot... yeah. Let's just say Ruin isn't very fond of watching Miku trying to take his lover away from him Right In Front Of Him thus making Ruin let his Yandere side out later on. No one cared about Miku anyway right? She won't be missed. Plus by that logic Ruin is doing the entire Celestial Family a favor here anyway. Miku won't be bothering ANY of them again in this Yandere Ruin AU is all I have to say. So I hope you guys enjoy. ;3
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~ Ruin's POV ~

It was I'm pretty sure a Saturday or Sunday all I knew is that the Pizzaplex was closed right now. So I've been helping Solar and Moon in the daycare's gift shop. Solar and I were telling Moon what we needed more of so he would order it to resupply. I had one side of the gift shop covered while Solar had the other.

While we were doing that the annoying blue-haired fangirl who says she's supposedly "Hatsune Miku" came in. I didn't pay much attention to her until she started flirting with my boyfriend. Solar tried to ignore her at first which he was doing really good at. However that didn't last for long.

In fact the more she flirted with Solar the more I would get pissed and would watch her close and very intensely. It only got worse when she sniffed him then even made a comment about having sex with him. She literally said "Want to go do something that will make you smell bad?"

When I heard her say that, that's when I snapped. I mean Solar himself as well as what she was implying so his "goods" as well as those "actions" and that "side" of him were meant for me and ONLY me.

Hearing Miku say that also made me think to myself "I think I have my next victim." However I didn't say that aloud for obvious reasons.

In fact I even waited till Miku left so I could go after her. However what was very satisfying to me was before she left Solar snapped and yelled at her.

Seeing and hearing Solar yell at Miku like that, I won't lie made me blush because goddamn that was fucking hot. It was so nice to see Solar prove that he as my boyfriend wouldn't take anything from her and would never date her or even anyone over me.

Just hearing and seeing that in general made me feel all warm and fuzzy on the inside. It was even to the point I had to hold back from shutting my eyes and placing my hands over my chest.

Either way as Miku left I snapped out of my blush as well as trance. I then continued to stare Miku down with murderous intent probably clearly shown on my face.

Ruin: *says to under his breath* "Oh Miku Darling, I fear you made a grave mistake my dear~"

Ruin: *still says under his breath* "No one can have my dear Solar Darling besides myself. He's mine and forever mine. You're demise will be very shortly dear Miku~" *chuckles to self*

Solar: "Ruin?"

Ruin: *snaps out of it* "Yes Darling?"

Solar: "You ok? You've been awfully quiet since she... *gives an annoyed growl like sigh*... she came in and flirted with me."

Ruin: "Oh! I'm fine Darling. Don't worry about me."

Solar: "Alright."

Ruin: "However, I did just realize I have stuff to set up for my performances in the theater tomorrow. So do pardon me while I take my leave."

Solar: "Moon and I could come help you."

Ruin: "Oh no no no. Nonsense! I don't need help. Besides, you and Moon are needed here and elsewhere anyway. I can do this myself. It's very minimal."

Solar: "You sure?"

Ruin: "Positive."

Solar: "Well in that case, sure."

Ruin: "Thank you. Good day to you Solar Darling, Good day to you too dear Moon."

After that I take my leave and head off to find the hussy whom I just watched try to steal my man in front of me. I just needed to have a few words with her.

~ Solar's POV ~

Watching Ruin leave I then hear Moon come up behind me.

Moon: "Is he always that suspicious? Like I've known him for a little bit since I came back from being dead but I don't know. That interaction and how suddenly he decided to leave like that seemed more off than usual to me."

Solar: "Eh, that's just Ruin for you."

Moon: "I know but still."

Solar: "What?"

Moon: "Did you not noticed how he was staring down Miku that entire time she basically flirted with you?"

Solar: "He's just being a protective boyfriend is all."

Moon: "That didn't look like he was being "protective" with the way he looked at her. Solar, I think you may have a Yandere boyfriend."

Solar: "Yandere? Ha! No, he's just protective is all. I mean I'd be protective of him too if I just witnessed him going through what I just went through."

Moon: "Keep telling yourself that. I've just seen that look he had many times before but with myself. He had murderous intention in his eyes while staring down Miku."

Solar: "So what?"

Moon: "Do you not realize how bad that is? He's probably going to kill her."

Solar: "No he's not. Plus say he hypothetically will kill her, does it really matter? He'd be doing all of us a favor anyway. I mean she'd never bother us again."

Moon: "I mean, you're not wrong."

Solar: "Exactly."

Moon: "Yeah. But murdering someone is still wrong."

Solar: "Haha! Oh that's funny."

Moon: "What?"

Solar: "Moon, you're one to talk so you speak for yourself. I mean, in the past under the influence of your killcode you've murdered so many people with no hesitation or second thought. Kids and adults alike mainly kids. Because believe me, I'm still an Eclipse. I lived inside a Moon before I was left behind the same way your Eclipse was. So I know."

Moon: "Touché"

Solar: "That's what I thought."

Moon: "Anyway, after we help out these customers, I need your help in the lab with something. Mind coming with me?"

Solar: "Sure."

With that we finished helping the costumers that apparently snuck in. After that, we headed out of the gift shop and headed to Moon's lab.

~ Ruin's POV ~

After I left the gift shop, I went to find Miku knowing she didn't get far. In fact she didn't. She was still in the lobby of the daycare. So that's when I put on my charm and manipulation act to get her to fall for me. It wasn't very hard.

Ruin: "Miku Darling~"

Miku: "Oh, calling me "darling" how romantic."

Ruin: "Anything for you Darling~"

Miku: "Anything?"

Ruin: "Of course."

Miku: "Mind trying to help me win over Solar then?"

Ruin: "But why need Solar... *pins her against a wall*... When You Have Me Instead~"

Miku: *blushes* "O-Oh my. I... I didn't know you had it in you Ruin~"

Ruin: "There's A Lot About Me You Don't Know~"

Miku: "Clearly~"

Ruin: "Plus, may I say do you look rather stunning today my dear~"

Miku: "O-Oh... Th-Thank you."

Ruin: "Of course~"

I then hear her give a little bit of a giggle. My plan for making her fall for me just so I could quite literally back stab her was going just to plan.

Ruin: "Now... mind going out with me somewhere? Somewhere private so I could perhaps get to know you better~"

Miku: "Well... if I can't have any of the celestials, at least I can have one~"

Ruin: "Exactly~"

Miku: "So sure. Where are you thinking of taking me?"

Ruin: "I know of a place."

After that I pulled away from pinning her."

Miku: "Alright. What time do you want me tonight?"

Ruin: "6pm will work just splendidly Darling~"

Miku: "So in about an hour and a half from now."

Ruin: "Correct."

Miku: "Got it. Where do you want to meet?"

Ruin: "Here, in the main entrance of the Pizzaplex. I shall take you to where we shall go from there."

Miku: "Alright."

After she said that, she left and so did I. However, with what I had planned I had to cover up before "the date". So I hacked the cameras in the vicinity of the area I'd be with Miku.

I basically hacked the cameras to stage them to look like they just glitched out and shut off. This was only to restart automatically after I was done with the plan.

Once I got the cameras rigged, I went to the kitchen and collected the largest, sharpest, knife I could find. I then grabbed a garbage bag knowing I would need one of those as well.

After getting what I needed I kept both of these things with me and hid them out of sight. This was in order for me to use them later tonight when the right time finally came.

- Time Skip -

It was now 6pm and just as I was expecting, met up with Miku in the main entrance of the Pizzaplex.

Ruin: "Ah, alas there you are Darling~"

Miku: "Yep. Where are we heading?"

Ruin: "Just follow me Dear Miku~"

Miku: "Lead the way hot stuff~"

Hearing her say that made me quietly gag to myself. In fact it made me almost even throw up a little in my mouth. You see, I may be pansexual but ever since I started dating Solar I couldn't even dream of being with someone else that isn't him.

Either way I still tried to act as if nothing was wrong. Luckily I was a good actor and Miku was just an easy person to manipulate. So because of that I continued to play out the plan.

Ruin: "Of course."

After saying that she nods and I started leading her through the Pizzaplex.

Miku: "That's odd. Shouldn't we have just left through the front doors? It would've been easier."

Ruin: "Nonsense. I know a secret way to exit."

Miku: "Alright."

With that I lead her through the Pizzaplex and to the very bottom behind parts and service where it was dark, had no cameras, and no one ever comes down here.

Miku: "Ruru, I... I don't like this anymore. This isn't a date. This is scary."

Ruin: "Nonsense darling. We're almost there."

Miku: "I don't know about this Ruin."

Ruin: "Darling, it's fine. The exit is right through this door."

Miku: "Finally."

Ruin: "Of course. Now... *opens the door and holds it for her*... after you Darling, ladies first."

Miku: "Aw, so gentlemen like. Thank you."

Ruin: "You're welcome."

After she walked inside, I immediately dropped the act and stopped acting as a whole. I walked in after her and locked the door tightly.

Miku: "Ruin? Why'd you lock the door?"

Ruin: "Oh, I Simply Just Want To Have A Little Fun With You~"

Miku: "Ooooh I get it. Ruin You Dirty Man~"

Ruin: "Oh, not that type of fun Darling~"

Miku: "What?

After saying that, my sweet friendly smile dissipated and I grew serious with a sinister look instead. I then took out the knife I was hiding this entire time.

Ruin: "Do you recall how you flirted with Solar?"

Miku: "Yes?"

Ruin: "Well... *chuckles*... You made a grave mistake flirting with my dear Solar Darling... In Front Of Me!"

Miku: "What!"

Ruin: "I never loved you. I did this so I could simply murder you."

Miku: "Wait... What!?"

Ruin: "You see my dear Miku darling, no one is allowed to have my dear Solar Darling besides myself. He's forever mine and will only ever be mine. No one will ever take him away from me. I won't let them."

Miku: "You Psychopath! You Manipulated Me!"

Ruin: "Well, mayhaps I did. Haha! Silly little me~"

Miku: "Ruin! How Could You! I Thought What We Had Was Special!"

Ruin: *scoffs* "We never had anything Darling.  You are nothing more than a stain on the rug to me."

Miku: "You're sick and twisted."

Ruin: "I know. But so are you. Doing all you've done to my friends, if I can even call them that."

Miku: "They will love me!"

Ruin: *scoffs* "No they won't. They never will."

Miku: ....

Ruin: "Plus, on top of what you did in the past and what you did just earlier today with attempting to steal my boyfriend in front of his actual boyfriend. How prosperous. How dreadful. How horrific. You disgust me."

Miku: "You manipulative psychopath!"

Ruin: "Oh... I am."

Miku: "I should have never trusted you!"

Ruin: "No. You probably shouldn't have."

Miku: "I knew this was a bad idea from the start!"

Ruin: "Indeed it was."

Miku: .....

Ruin: "Yet even if you didn't trust me, I would've killed you either way for flirting with Solar and trying to take him from me."

Miku: "Listen! I didn't know Solar was taken! I didn't know you two were dating!"

Ruin: "Well... alas now you do."

Miku: ....

Ruin: "In fact, now that I think of it, Solar would never have interest in the likes of you no matter the circumstances. He's made it quite obvious he's gay many times before."

Miku: "Again! I didn't know that!"

Ruin: "Tisk tisk. What a disappointment."

Miku: "Please. If you just let me go I won't go anywhere near you or Solar again."

Ruin: "Too bad. You may beg all you please darling, but alas that still shan't change the fate you have my dear."

Miku: "Ruin please!"

Ruin: "Sorry darling, you aren't getting out of this."

Miku: "Fine! Fuck this shit I'm getting out of here on my own!"

She then runs around me to the door and tries to open it but it wouldn't budge.

Ruin: "No use in trying to run or cry for help dear. I've locked the door and have the key. This room is far too below the Pizzaplex for anyone to hear you."

Miku: ....

After that I go up to her then grab her from behind pressing her back against my chest. While my other arm held the knife pointed with the blade at her chest. I did hesitate for a moment though as I held her.

Ruin: "Hm... tis' what a shame for me to have to go and ruin such a beautiful perfect pair of bosoms."

Miku: *blushes* "What!"

Ruin: "Listen, I may be dating Solar but I am still Pansexual my dear. So it should come as no surprise that I still admire the beauty of the female body just as much as I can admire anyone else's."

Miku: "I get that but it's still so random!"

Ruin: "I suppose that was quite out of the blu for me to say. Perhaps even inappropriate."

Miku: "You think!? Especially with the current circumstance I'm in!"

Ruin: "True. I suppose I just accidentally spoke my mind aloud there. So do pardon my behalf darling."

Miku: ....

Ruin: "Really, my sincere apologies dear Miku. I will still at times admire the female body and don't mean to speak it aloud. I mean... I do ever so often date Luna in a Vee relationship, Moon being her main partner while Solar is mine."

Miku: "What? You Yandere for Luna too?"

Ruin: "Oh good heavens no. The only person I shall ever be considered Yandere for is no one but my dear Solar Darling."

Miku: .....

Ruin: "Now, where was I? Ah yes, I remember."

With that I move the hand that held Miku against me to covering her mouth with it. I also still held her with her back against my chest. The only reason I covered her mouth was for extra precaution incase anyone by chance passed the room I was about to kill her in. I didn't want to blow my cover just because of a scream due to bloody murder.

Ruin: "Hush now, don't fret my dear. I assure you, your death will be quick but painful. I shall promise you that."

Miku: *says in a muffled tone* "You fucking insane Yandere!"

Ruin: "I know I am."

Miku: *still muffled* "Fuck you!"

Ruin: "Goodnight dear Miku. I wish for you to have a blissful eternal sleep darling."

With that I plunge the blade of the knife deep into her chest. Once I did that she gasped for air a little then her chest fell flat as she stopped breathing when her heart stopped beating.

End A/n:
And I'm ending this oneshot here. However this isn't the end I am planing on making apart 2 because this part was getting too long. So look out for that. I will give you a hint that next part will continue with the after math and clean up. It will also have a cute Solar x Ruin moment. Anyway, I had a lot of fun writing this. I especially love how absolutely evil I made Ruin in this. I mean come on, Ruin manipulating Miku into liking him so much that he even successfully set up a date with her the same night only to murder her. Now that's fucking diabolical huh... like, goddamn.

Other than that, I hope you guys liked this oneshot and thought it was good. I haven't written with Yanderes in years so I thought I was kinda rusty. I'm pretty sure the last time I even wrote with Yanderes was when I was in my Aphmau phase back in 2019 or even 2018 with Ein. So yeah, it's been a long hot minute. However, I do know I got better at writing since then considering I didn't find this Yandere thing alone as cringy as I now find my Yandere Ein books. In fact I'm proud of this one.

Other than that, I again hope you guys enjoyed this oneshot it was approximately 3150 words in all by the end of it. So until next time...
"Stay Awesome, Stay Safe, Have A Great Day Wherever You May Be And Remember I Love You All And I Will See You Guys Escalater Peace Out My Pups." ;3