Blissey will be a Supporter Pokémon that can heal and increase attack speed. The Unite move will also help out your teammates in battle.
I would also imagine Blissey will have a lot of HP and take a while to knock out. We don’t know if Blissey will have to evolve from Happiny and Chansey, or just be available right out of the gate.
The developers have also promised Blastoise in the game, but have not given a time frame for that addition.