It looks like KAY/O grew up with Brimstone before becoming a robot. They played the same sports and worked the same jobs together as well.

Why do I think it's KAY/O? Even before the VERSUS // KAY/O+BRIMSTONE card was released (See next pic for reference, it looks like they're fighting together), it was already evident that KAY/O and Brimstone had some form of friendship before.

There's also a few other messages in there, I'll get to those on another post.

[I found this in the new room in the range. If you want to take a look you can go down the stairs next to the shooting test]

VERSUS // KAY/O+BRIMSTONE card for reference