Samantha's late love, held up by her as the standard of devotion Carl failed to meet.
Charlie appears only in Samantha's lecture to Bautista about passion. She declares that a woman likes a man who fights for her with everything he has, and that this is why she and Carl broke up — they had great physical chemistry, but he could not even die for her the way Charlie did.
The comparison is the whole of Charlie's role here: the dead man who proved his love the way Samantha thinks love should be proved, invoked to shame a Tigran into recklessness on the eve of an assault.
He is Samantha's benchmark for devotion, cited to needle Bautista and Carl.