Pretty shocking fact I found out just now in Alchemy Stars, spoilers for Mia's personal story that I dunno if it'll go anywhere
Mia is some long lost sister of Carleen. But I guess she was abandoned as a kid for whatever reason and led a completely different life, seemingly has lost her sanity along the way. In her personal story, she looks out for Carleen when the MC missed out on a chance to ask Carleen on a date. I didn't pick up on why but yeah it's because Mia is her sister. Ohh, in Carleen's bio at max affinity, it says she lost her mother and sister in some war, but I guess she survived but with various prosthetics
Oh dang, unlocked affinity 4's file for Mia, and yeah that one confirms it. She joined the knights to help out her dad and presumably got lost in action in that second Darkover Crisis thing, which is why Schwartz is really reluctant to let Carleen follow her suit.