There isn't too much to say about Cullen, mostly because even his fans know so little on the DI that kept Sun Hill on its toes. Part of Manien's "New Sun Hill", Cullen knew Chandler from working with him on the West side, better than the rest of the relief. He kept out for his own agenda though, a very proficent officer. He warned Chandler against dating Spears as Chandler was up for Bourough Commander, and "He couldn't be seen messing around with pretty young DC's".
When numbers, there were, Cullen sided with Chandler for as long as he deemed safe, then getting a transfer out to MIT along with DC Duncan Lenox.