I'm torn between Grim and Elaris.
With Grim, I like the idea of Ratchet having a father-figure type character. I love it when a badass character has a doting parent looking out for them.
Elaris was one of my favorite new characters, though. It's about time they introduced a character who can nerd out on the same level as Clank, and, between her and Cora, I liked Elaris way more in the game and movie. Cora just felt like a replacement for Talwyn. Only difference being Talwyn's initial hostility was understandable, and she became nicer a lot faster.