Supposed to be angst... that didn't come through as strongly as I'd like. Additional notes at the end, please read them. No X Reader for Teucer.

I'll Find You (Teucer)- 9.19.22
    "It's been seven years since my big brother, Ajax, stopped writing home. His letters had been getting more and more spaced in the year before he vanished, going from once a week to once a month. Six months before they stopped completely, there were only three more. One came four months before he vanished, one two months, and then the final letter came just before he'd vanished. News spread not long after that Tartaglia had gone AWOL from the Fatui. I'd found it odd, because I knew that Tartaglia was the alias my brother used as a toyseller. Turns out yes, my brother was one of the best in Snezhnaya... one of the best soldiers. I'd been lied to by everyone. Even my own brothers and sisters knew.
       The Fatui claimed he was dead about a year or two after he went missing. I believed it, but then... I learned something. It was the tiniest slip of the tongue from a diplomat who'd worked for my brother. While she was offering her condolences, she requested to see her master's grey vision. I told her we didn't have it, and she explained it must not have reached us yet. It never did. My parents didn't know my brother was a hydro allogene, so they never pressed on the subject. When a fatui soldier with a vision dies, the empty vision is given to the family.
        It was at that moment, I knew. The Fatui lied. I'm not going to stop or go home until I find my brother or his greyed-out vision." Teucer explained, taking a sip of his tea from where he sat at the Wanmin restaurant. The tea had long since cooled, not that it bothered the redhead too much.
          "Holy shit bro, that's quite the origin story." The blonde adventurer poured himself another cup of oolong tea, swirling the teacup before taking a sip and leaning on his hand.
         "And what about you? Gonna explain why I dragged you out of a mob of angry guild members? Or even your name? Oh, or your accent being from Mondstadt and not Liyue?" Teucer prodded, polishing off his tea.
        "Heh, you got me." The blonde adventurer put his hands up in a surrender. "I suppose I owe you as much after you saved my ass back there. The name's Timmie."
         "I'm from the city of Mond, accent gives it away. I was never a liked child, people making fun of me and outcasting me for standing up for the pigeons was pretty much my childhood. Same reason you pulled me out of those stuffy adventurers, hell, I should've just joined the ordo. People apparently don't value the feelings of animals, but at least in the ordo I'd be protected by the cathedral." Timmie sighed as he poured himself another cup of tea. "Well, I owe you big time after you saved me, so why don't I help you look for your brother? I mean, I'm not perfect, but I've got a good deal of experience under my belt with exploration and tracking; It was part of the Guild's official training, after all."
          "You really think you can help me?" Teucer questioned before he shrugged. "I guess it couldn't hurt. The Fatui claimed the case was closed after they gave up five years ago."
          "The case is closed, it's been seven years now total, how do you know he's still out there?" Timmie asked, rising from the table after he and Teucer split the bill. The two walked down towards the gates to leave the harbor. "If the trail is cold, what evidence do you have?"
          "Harbor records that I had to claw to get. My brother's name, or names, actually, weren't listed on any in the last seven years. The last mentions of Tartaglia in any records of Snezhnaya, from what I could get access to, stop around six years ago. Ministry of Civil Affairs paperwork, from what I could get from that kind adeptibeast, stop even earlier than the Snezhnayan ones." Teucer sighed. "It was disheartening. I was about to give up, when I found a peculiar lead. A small, seemingly insignificant, narwhal handkerchief. It had red seeping onto it and was tied onto the arm of a short male, freshly bloodied. I saw the man make his way in the direction of Wangshu Inn after he'd stopped to speak with the kind Adeptus in the city, Ganyu, he called her."
              "And you believe your brother... made the handkerchief?" Timmie questioned. "They hardly seem like related events."
               "I have a matching one." Teucer added, revealing a handkercheif decorated in ruin guards. "My brother made this for me when I was a kid. I've been carrying it around for good luck."
               "I used to have an imaginary friend like this when I was little. I'd read about field tillers in some of Librarian Lisa's forbidden section of the library." Timmie smiled, albeit the grin was laced with a forlorn look. "She eventually gave in and let me read about Khaenri'ahn history."
               "I called him Mr. Cyclops," Teucer admitted with a soft smile, reminiscing about his childhood. "My brother went to extreme lengths to make sure I believed he was a good guy. Sent me commissioned action figures, made me a quilt for my bed, even took me to what I thought was Mr. Cyclops' headquarters. It wasn't until I was older that I learned that we'd broken into a Fatui Harbinger's lab. Heh, and completely trashed the place, too."
             Teucer fiddled with the fatui badge on his jacket. "I managed to fish a little bit of intel out of Katheryne, but man, was she hell to hack into."
               "Hack?" Timmie's eyes blew wide. Of course, Teucer wasn't exactly scrawny, but he had a hard time seeing him tacking a blade and hacking into poor Katheryne. "Why would you do that to Miss Katheryne?! Wait, when did you do that? She was fine this morning!"
                "Relax, I had her rebooted as quickly as I got her taken offline. Lady Sandrone'll probably kill me for it, but I can probably shake her off, especially if I bring her back some nice mechanical specimens or something." Teucer dismissed the idea of his boss coming at him rather quickly.
                "Dude. Slow down, back up." Timmie crossed his arms. "What do you mean with 'offline' and 'reboot'? And who is Lady Sandrone?"
              "Simply put, Lady Sandrone, or the Marionette, is one of the eight Fatui harbingers and my boss." Teucer responded, though his cerulean eyes were looking elsewhere. "Katheryne is just one model of her many bionic robots. Ever wonder why there is a Katheryne in every region, and they're exactly alike, no matter how much time passes and what happens to them? Really, with how little people trust the Fatui, you'd really think the Adventurer's guild being based in Snezhnaya would be a huge red flag."
              "No matter. You want to help me, let's move." Teucer started, adjusting his bag and slinging it over his shoulder.
***
           "We shouldn't be here-" Timmie started, quickly cut off by a very angry Yaksha, the very same 'short man' Teucer had been tracking.
              "Damn right you shouldn't." Xiao growled, the handkerchief on his arm having been replaced by a sturdy dressing, though the two-tone turquoise hair was hard to miss. He crossed his arms, a spear resting on his back. "Mortal beings in Jueyun Karst, what is wrong with you two?"
               A field tiller, well, three field tillers, came around the mountain side. Xiao, despite his displeased disposition against the humans, quickly moved to protect Timmie and Teucer, only to realize the ruin guards were aimed at him.
               He grit his teeth and let out a growl, slashing the ruin guards to no avail. Who in Teyvat was controlling these? Ruin Guards were known to be sequentially linked with each other, but this was advanced even for Ruin Guards. And the fresh silver plating, the weak points were protected, these weren't your typical Khaenri'ahn spoils. Someone had deliberately upgraded and reinforced these behemoths!
             "I will have order!"
              "Riptide!"
             Two voices rang out, saving an overwhelmed anemo yaksha, though he'd never admit to being overwhelmed. Two adepti made their way down the mountain slope, one with dragon features and the other resembling a... narwhal? That was odd, thought Timmie, the only living hydro adepti-like creatures were the sealed away Osial and Beisht. The two adepti landed gracefully, surveying the damaged ruin guards.
             Teucer bit the nail of his thumb, a bit nervous, subconsciously muttering out loud. "Lady Sandrone is going to kill me!"
               That seemed to get the attention of the narwhal adeptus, who turned to face the two boys, his eyes widening as he ran over and lifted Teucer, twirling him around with an elated smile. "Teucer!"
                "Ajax?!" Teucer's shock faded as he hugged his brother's neck. Childe seemed to have no trouble holding him, despite him now being nearly the same size.
              Childe set his brother down and got a good look at him. Teucer had grown so much since he'd seen him last. He didn't forget how long he'd been gone. Teucer was seventeen now, tall and lean, with a rather fluffy mane of bright red hair. He was well dressed, and familiarly dressed at that. His hand paused as it grazed over the delusional eye snowflake.
             "Oh, Teucer.." Childe sounded distressed. "Please tell me you didn't..."
               Teucer looked up, noticing the sparkle in his brother's eyes. When had those come back? They'd been dull as long as he could remember. "I did what I had to. When no one else had answers, the Fatui let me into their ranks. Lady Sandrone was the one who allowed me access to the, erm, now destroyed, field tillers."
             "That was you!?" Exclaimed Xiao with a growl. He was quickly grabbed by Zhongli, who prevented the short yaksha from attacking Teucer like a rabid raccoon. "You purposely upgraded those damned metal behemoths?!"
             "Well, I didn't plan on only needing to explore one region of one nation and finish my job." Teucer shrugged, enraging the yaksha more as the redhead ran his hand over the splintered metal, attempting to repair what he could on the spot.
              "You were going to tear through Teyvat with these... these..  Metal Leviathans?!" Xiao screeched in a rather birdlike manner.
               "One does not think you are using the term 'leviathan' properly." Cloud Retainer corrected as she landed on the ground by the former Geo Archon, stirred from her abode by the Chaos. Surely Mountain Shaper was angrily watching from his amber-decorated mountain, where he was likely woken abruptly from his nap.
                "Irrelevant," Zhongli butt in before an argument, though Cloud Retainer would most certainly win, could spawn. "Now, Teucer, why don't you and your friend come with Ajax and I and we can talk this over calmly and maturely?"
               Teucer nodded and took a step forward, dropping a device on the ground to come back to later. A beacon of sorts that he'd developed during his first few months in Sandrone's grasp, this poor harbinger wasn't sure if she liked or disliked how intelligent the former eleventh's brother was.
***
             "Now, what brings you two to Liyue?" Questioned Zhongli, sitting at the table across from the two foreign boys. His long, earthy brown tail was wrapped around Childe's waist, the fluffy end resting in the ex-harbinger's lap.
             "It's been seven years, Ajax. Everyone thinks you're dead. Heck, I'd think you were dead if it weren't for Miss Ekaterina." Teucer responded. "How could you lie to me? I wouldn't have judged you for your real job- I probably wouldn't have been so intrusive when I was here last."
               "Teucer, you were ten. You were a bright-eyed child." Childe sighed, he paused. "Wait, what about Katya?"
              "Well, you never told anyone about your vision. I only knew about it since I saw you use it when you'd taught me to fight to protect Tonia... That last time you were back in Snezhnaya. And since you never really told Mama and Papa, they never waited around for the greyed out vision we never got. Miss Ekaterina asked to see it, and when I told her that we didn't have it, well, you can imagine it never arrived." Teucer crossed his arms. "I joined the Fatui to do some digging, turns out they really don't have an age restriction. With Papa's permission, I was in the ranks at fifteen. I didn't even have to do much digging once I made it into their ranks. Pretty hard to miss the records of a dead harbinger. Lady Sandrone took me on, let me have access to Lord Dottore's old scraps, and I thought I made some decent prototypes. Obviously I didn't account for adeptal attacks."
                "Teucer, I'm sorry, I really am." Childe looked down. "I told Mama and Papa not to tell you. I mean, I was fourteen when Papa tried to put me through military training, you were just a toddler. Not that I ever blamed him... You were a toddler, Anton wasn't even in gradeschool yet, Tonia was just starting school, my point is, I was aggressive. And you were all too young to have that danger in the house. But I didn't ever want you to fear me, Teucer, so I thought about it. What was the best job a kid could ever imagine? It'd have to be an amazing job to justify me working overseas."
                "But what happened?" Teucer sighed and looked up at Childe. "Why did you just disappear? And why do you look like a story-book adeptus?"
                 "In the short definition... I am part Adeptus now, a good excuse to shy away from the Tsaritsa." Childe answered.
                  "Even if I've told you, she'd throw a minor temper tantrum and get over it, Ajax." Chimed in Zhongli.
                "My point is, it was better to stay here, not try to reacclimate myself with frigid Snezhnaya." Childe reasoned, resting his hand on Teucer's. "And look at you. You've grown up. I'm so, so proud of you."

Further Notes:
   To clear up why Childe is now an adeptus, in short, Zhongli tried to purify him of the abyss and because the abyss had such a deep root in Childe, he nearly killed Childe in the process. Using a last gift from Guizhong, Childe joined the adepti in the form of his spirit animal, a narwhal. Childe mentioned that he wouldn't adapt well to Snezhnaya because now he is constituently made of hydro energy, not just being a vision bearer. Because the abyssal taint was removed, that is why Childe is described as having a sparkle in his eyes again. Zhongli calls him Ajax, even in front ot Timmie, because he has given up the name Tartaglia and the codename Childe as he is no longer associated with the Fatui.
     Teucer was initially not going to be part of the Fatui, but it would make more sense for him to be a part of the organization because being a fatuu would grant him access to more information. He is under Sandrone and not Dottore because I felt as though he would have more access to mechanics under Sandrone, but he got a lot of the pieces from Dottore. Dottore has obviously dabbled in Khaenri'ahn technology more than once, that much is backed up by the game and manga, but it is shown that he also dabbled in human experimentation. Sandrone, who is shown to still actively be using puppets and robots, made a lot of sense that she would be interested in applying the uses of Khaenri'ahn war machines past just Dottore's game of trying to attain more and more raw power. I eventually chose Teucer to work for her as well due to the irony that Childe said that Sandrone always looked like she strongly disliked him. It also made a lot of sense with her leaked design that Sandrone would have allowed Teucer to do whatever he pleased as long as he brought her back something mechanically relevant.

Oh, and Xiao was supposed to be serious, but he came off more as a scruffed cat.