In Amphitrite City there is a Master ball but don't get excited because as far as i know there is no way to get to it :(
This guy trades you a Delta Shuckle for a Bulbasaur
Home to the Seventh gym of the game, Amphitrite City the gym is rain themed, so bring a electric or grass type. the leader has a phione or manaphy before this gym you can catch mew. after, the gym leader is convicted of not wanting to take a (Useless) shot so Malde does not infect the town. then you head up to the rose crater.
you witness Jaern of the sky cult battling a Giritina looking cultist. The cultist defeats Jaern and then defaces him. then you proceed to where you met mew while battling Jaern for a final time. the Giritina cultist has you use hyperspace hole on the wall enter the gauntlet. before you go, you will be capturing Giritina in the gauntlet. bring loads of pokeballs.
after being champion once
go to where you meet mew and King Veysern prompts you to beat all 7 of the timeless but unlike the elite 4 you can leave and heal your Pokemon. the timeless includes: Cynthia, Alain, King Veysern, Steven, Dawn, a developer for this game, and their leader is Red
2 champion titles I think
you can catch Victini! go to exactly where you meet mew and it will be there