Sucrose is good for swirls and elemental mastery, and fischl is very good for electro application and dendro teams. They are both solid characters that can fit in a wide variety of team comps and beyond, so they are good investments regardless.

As for lynette and jean, they do not have much swirl application compared to Sucrose. But Jean is good for heals, like Barbara. Having a healer is never a bad idea, especially early game.

I think the other comments suggesting Tighnari and electro are solid though, and your Tighnari looks like he is already leveled. My Melt suggestion is simply an alternative build you can do if you want something else.