Complex sentence: Worried that she may scare Caitlin, Samantha decides to leave with Mitch, who's partner Trin had previously discovered, being the first to do so, some items and contacts from Samantha Caine's past.
Split into simpler sentences: Worried that she may scare Caitlin, Samantha decides to leave with Mitch. His partner, Trin, had been the first to previously discover some items and contacts from Samantha Caine's past.