That is a lot of overlap! Ilias was probably in the Circle from age 7-19ish and then close by in the Necropolis. Man I hadn't counted on him having many friends in the Circle but apart from a little jealousy friction I feel like they could've been quiet nerd pals?? Ilias was more of a goody-goody and cautious about breaking the rules, but like. he's fine with alarming people and pretty easily swayed, Casimir seems nice enough on a personal level so it's probably fine, those rules are probably meant for other people?? Etc. etc. Especially in his late teens, forbidden intellectual curiosity was Ilias's low-key teenage rebellion of choice, that's how he ended up falling for a sociopath and ruining his life, so /jazzhands

ON WHICH NOTE uh, depending on how much drama you feel like having on hand, Casimir might also have been familiar with Ilias's ex?? who I need to name. he was a mage with some similarly alarming research interests, though probably a more cautious monster, who was around Nevarra City for a year or so before getting abruptly transferred.

Either way: perhaps friends, perhaps Ilias abruptly cut off contact after his cousin's death, and then Cas was transferred?? And now: this.