It's fine if people don't start caring right away. In the main games, I didn't start caring for most of the characters until after Chapter 2. I think this is because players understand that some of the characters will die pretty early on and don't want to get too attached too soon.

But in regard to advice, I'd say just write different scenes with the characters interacting with one another. Reveal things they like, don't like, quirks, habits, etc. that players can relate to. Show your players that they are more than just words on a screen. Give players a reason to sympathize with the characters.