He and Brennan dated for a short time, but things moved a little too fast for her when Sully asked her to sail to the Caribbean in a boat he named after her (you know, like people do).
He was an FBI agent who stepped in as Brennan’s partner while Booth was in therapy.
After the death of his partner, Sully set out to live life to the fullest, acquiring a number of hobbies and making spur-of-the-moment decisions.
The character was barely mentioned or seen since then, but now he returns with monkey wrench in hand and looking for some bone.