Both Leena and Robin are overall great characters to have, especially early on.

Leena is the best dedicated healer in the game. The only reason she isn't on S tier is because she lacks the versatility to perform other roles, but she shines on boss fights and hard content in general.

Robin is both a great physical DPS, and the only available unit with an item drop rate increase trait. This automatically turns him into the best farmer in the game, so he's really good to have on your roster.

Because of the Conquering system, the earlier you get a Unit, the more souls you'll be able to get for them by placing them on Conquer slots.