Possible to change if he's made Canon.
Buster Freeze is the Representative of LuxrayBlast Koritsu; in MentalTale, he's sick and has to stay away from his home in the Orlan Region. He misses his Mommy Mizuki and Daddy DeepFreeze, but, knows they visit him a lot. He wants Frisk to help him get better so he can go home, he wants to leave the craziness before losing his mind.
He wants to be good friends with Derpy, but, everytime she comes to see him, he sees Luca spying to make sure Buster doesn't get too close to Derpy, which scares him & he can only Thank Derpy for bringing him his meals. It scares Buster to think that if Blue found out that Luca was outside his room spying on him when Derpy brings him his meals, that Luca would get in trouble. He also knows that Luca hates him & he would be harmed if he said anything other than "Thank You" to Luca's Senpai. Buster's scared of Luca more than Derpy.
Buster's Story Introduction (Non-Canon)
My name's Buster Freeze, I'm a Shiny Flareon. Ever since I was brought to UMH, I've been trying to make friends, but, only one of them actually cares. Her name's Derpy. She always brings me meals, but, for some reason, Lucario, thinks I'll take her away from him, but, I won't. He's always watching as she gives me my meals. I wanna go home, but, I'm afraid that if I say anything else to Ms. Derpy, Lucario will strike. I only want to befriend her, but, it doesn't matter to him. I miss my family. Though, I may lose my mind if I don't get better.
After Lucario's Death
Buster, scared to even leave his room, sees even though the hallway is clear, often goes to Lucario's room to see him forever in his Hourglass starting to miss every time Derpy brought him his meals, seeing Luca out there protecting their relationship, but, Buster cries seeing the Hourglass with Luca's remains, the dust of someone who was stalking him, fall to the bottom of the glass, just so he can flip it and it fall again. Derpy takes him back to his room not cry for Luca when he was stalking him, just to get better in his own room.
Art by TehShinySylveon on Deviantart.