>>1422040
Aru is really good and Shun is a must for pvp if you care about that. Tsurugi and Neru are okay but they are tanks anyway so they won't deal that much damage. Never used Sumire but i think she is pretty irrelevant...
The best students in the game are from limited anniversary banners i'm afraid.
Supports give like 10% of their attack and hp stats and 5% of the rest to your main students as a buff. But i might be wrong on the exact numbers.
But lvls also increase stats like healing so it makes Serina heal for more for example. Some support students also deal damage with their skills so their attack matters too.