Callisto and Peter Nicholas were a couple.
After Colossus's time with the Acolytes, he met up with her again, when Marrow was going to blow up bombs to try to get even for the Mutant Massacre. Peter gave her a big kiss at the end of the issue.
Callisto was sort of a nemesis for Kitty, if only by trying to make her marry Caliban that one time, in return for the Morlock Healer saving Colossus after he was nearly killed by the Brotherhood.