What do you mean Seijou isn’t the protagonists of Haikyuu!!?
Haikyuu!! I watched:
Season 1 first half: The introduction of the Great King and the leaves
Season 1 second half: Inter High Arc: plants kill the crows
Season 2 episode 19: Watch THE Ace slay
Season 2 second half: Spring High Arc: defeat is not the end
Season 3 second half: The return of royal plants in casual clothes
Extra (in manga): How’re the smaller plants doing