Out of the Wammy kids, Matt would 100% be Light's favorite.
In a domestic AU where L and Light settle in together and try their hand at "normality" and live on their own floor at Wammy's (clownverse by @ponury-grajek tbh), Light would probably lose his mind from the chaos.
L thought it might be good for Light and calm him tf down a bit, and he maybe wanted to get involved in the children's lives to see what parenting would be like.
But, it goes about as well as expected. The children figure it out within the week, and the successors confirmed it within three days.
Near and Mello make it their mission to figure out why L would bring Kira into their home base, while Matt knows immediately it's because their lips have been doing the tango.
Near and Mello would probably make Light's life hell, just make every interaction a nightmare. He gets progressively more and more wound up until one day he can't take it anymore and ducks into a random room and screams.
It's Matt's room.
Light immediately goes red, apologizes and is about to shuffle out in total embarrassment, but then Matt says fuck it and holds up a controller.
"Wanna play Mario Kart?"
Light, after playing with Ryuk after school for weeks, quietly just takes the controller and starts up a game.
Matt is chill. He is so unbelievably chill in this house of chaos it boggles Light’s mind. He has never met someone so passive in his life, and probably never will again.
Matt prattles on about what a pain Mello was being lately, how he's been insufferable to talk to and that he can't imagine how Light's dealing with the full force of it.
"Seems like a lot— just got off the Kira investigation too, right? While you were a student? And the suspect yourself? Sounds like a total nightmare to me, jeez. How'd ya do it?" Matt chuckles to himself a bit, blowing out some smoke. Wait, wasn't he like thirteen?
"I don't know myself..." He grumbled without thinking, immediately distracted by Bowser shoving him into a hole off rainbow road as he hissed obscenities at the online player.
"Ha! Not as pure as you make yourself out to be, huh?" Light was about to reply, but cut himself off with a shout when Matt shot him with a red shell right before the finish line.
Light starts joining Matt for Nintendo turned game nights when the irritating children start making him itch for his pen, or L has decided to be particularly obtuse that day, and even when he just feels like screaming.
For a barely teen, Matt was easy to chat with. Light didn't go into depth with some of his problems, but still Matt just snickers and agrees with him most of the time. Light knew he was probably getting probed for weaknesses, but he didn't care at this point. He needed someone normal to talk with.
They even delve into respective tech/hacking skills, argue over which government organization's files are harder to get into, what they're favorite code language is, etc.
Matt himself wasn't looking to run to Mello or Roger with their game night chats, it was nice to find someone to play with once in a while, no way was he gonna betray Light's trust for no good reason. Playing video games with him was hilarious, he got super serious and devious when he locked in, and his rage rants were prime comedic material. Not to mention that he's never got to geek out about hacking with anyone before. Who cares if he was Kira, he needed a reliable teammate for WoW.
Light’s favour for Matt starts showing itself outside the game nights. One of the younger kids stole his psp and broke it? Matt would find a newer version sitting on his bed in box. A limited release was coming up but his allowance was short? Light dipped into L's limitless accounts to get a copy for Matt's collection.
Ecstatic, Matt thanks him in the hall way with a big grin while the other kids look confused.
Rumor gets back to Mello, who is quite pissed off Matt has betrayed them to become Kira's favourite.
Matt just shrugs as Mello screeches, they'd figure out sooner or later that Kira was just a mildly pathetic, dry humoured, burnt out college student with a side of megalomaniac supervillain.
Until then, more special treatment from his step-Kira.
