I would say Ellie, she has no past experience of the civilized world and so it would be less difficult to comprehend the violent world she is in and it would be easier for her to mentally deal with the violent or life threatening situations she could face. But Bill is like my description, but a little more selfish.
I say Tish, because she's just so gangsta! God I love her. I watch that part of the trailer, when she tripped that surviver with a club, then threw down the club! That was gangsta! Or that demo at PAX, when Tish helped lift Joel up with one hand, up that ledge...that was so gangsta! Oh wait, or that other part, when she ran up on some infected clicker female, and shot her point blank in her head, twice..then just stared at her....that sh*t was sooo gangsta!