leo first appeared in the episode "the fall out", where he was nothing more than a simple doctor. however, he was promoted to professor after former professor sam ryan left. in the episode "ghosts", both his wife, theresa and his daughter, cassie are killed in a car accident. he began a relationship with fellow professor janet mander in "safe", but ended their relationship in the episode "domestic". leo often took a strong opinion on cases and often became emotionally involved. he died in the series 16 finale "greater love", when he sacrificed himself to save many other from a terrorist bomb explosion including nikki and jack. he was buried with theresa and cassie.