Sonja finds Undyne's house and sees Papyrus at her door.

"Papyrus!" Sonja runs and hugs him.

"I HEARD THAT YOU'RE HANGING OUT WITH UNDYNE! OH BY THE WAY, SANS WANTS TO GIVE YOU THIS." Papyrus pulls out a letter which it's a light pink envelope with a heart on it.

"Awww! So nice of him! I'll read it latter." said Sonja as she puts the letter in her pocket.

"Psst! Be sure to give her this!" Papyrus pulls out a bone with a red ribbon tied to it.

"She loves these!" Papyrus knocks on the door as Undyne appears.

"Hi, Papyrus! Are you ready for extra private one-on-one training?" asked Undyne really confident.

"YOU BET I AM AND I BROUGHT A FRIEND" Papyrus moves away which Sonja nervously waves.

"Sonja, glad you showed up." Sonja smiled as Undyne lets her inside along with Papyrus.

"HERE, UNDYNE! MY FRIEND BROUGHT A GIFT ON HER OWN!" Papyrus gives Undyne the ribbon tied bone.

"Uh, thanks. I'll put it with the others." Undyne places the bone inside the collection of bones that Papyrus gave her a bunch of times.

"So are you ready to start?" asked Undyne.

"WHOOPSIE DOOPSIE! I JUST REMEMBERED! I HAVE TO GO TO THE BATHROOM! YOU TWO HAVE FUN!" Papyrus leaps through the window, smashing it.

"What the heck?" Sonja asked really confused. Undyne noticed the pink envelope in Sonja's pocket.

"Hey, what's that in your pocket?" Sonja looks at the letter and blushes.

"U-U-Uh, it's nothing." Sonja stammered.

"Let me see!" Undyne tackles Sonja.

"Undyne, no!" Sonja pleaded as Undyne finally has the letter in her hand.

"Oh, please don't you dare read it!" Undyne uses a spear as a letter opener and opens the letter.

"Oh darn..." Sonja covers her face.

"Dear, Sonja.

I just want to say hi and whenever I get close to you, my soul beats faster than before. I think you're really enchanting for a Mermaid like you. I hope you feel the same way to me. 💙💙

-Sans."

Undyne smirks at Sonja who is blushing so bad. 

"Awww, someone's in love with a big boned skeleton." Sonja squeaked, covering her face.

"Do you like him?" asked Undyne.

"A-A-As a friend." said Sonja.

"You're trying to hide it huh? Well, if you meet up with Sans there's no point of hiding it anymore." Sonja sighs, blowing her bangs.

"But how should I tell him? I know he's going to reject me if I tell him." Undyne sighs.

"I'm not good with any advice but just be yourself and do what's right." Sonja smiles.

"That's actually, not bad." said Sonja.

"If you want to know more, you should go ask Dr. Alphys." Sonja stands up.

"Dr. Alphys?" asked Sonja.

"Yes, she's like a scientist and a anime fangirl. I have her here a bunch of times and she has a thing for me." said Undyne, blushing.

"Are you telling me that you're in love with Dr. Alphys?" asked Sonja.

"W-W-Well, I..." Sonja smiles.

"We can help each other. You and I will tell Sans and Alphys how we feel and I bet they feel the same way." said Sonja.

"Now you're getting somewhere!" Sonja smiles playfully punching Undyne's arm and Undyne does the same before sharing a laugh.

Outside of Undyne's house, Sonja is already on her way.

"So what are you going to do now?" asked Sonja.

"I'll be heading over to see Papyrus. Oh if you need to call me just give Papyrus a ring okay? Since we're in the same spot I'll be able to talk too! Well, see ya, punk!" Undyne dashes away which Sonja smiles before meeting with a river person.

"Excuse me, can you please take me to Hotland please?" asked Sonja.

"Sure thing, human." Sonja gets on the boat and headed all the way to Hotland.

(Here's Chapter 16. I hope you like it and I don't own anything.)