Y the Charizard, commonly referred to as Master Y is an immense Mega Charizard Y who is the leader of the Mega Force(a group of immortal non-Legendary Mega Pokemon tasked with protecting the Four Worlds), and represents his Team at the Storming Base as the only Mega Force member there(others visit from time to time). Since Mega Force members use their species name(i.e. Sceptile, Tyranitar), and there are two Charizard Mega Evolutions, Y's name comes from Mega Charizard Y.
Y is a massive Charizard even larger than Tai, his great-grandson(this is due to his age and experience in battle). Y has the appearance of a Mega Charizard Y, with the basic Charizard shape but with a longer third horn between the other two, larger wings with ragged edges, a long crest on the outer edge of each wrist, thinner feet, a different-shaped muzzle, a spike near the base of his tail, and three downward-pointing rides on the end of his tail.
Possibly due to his age and experience, Y is a wise, calm, kind leader. Despite seeing many major events and fighting in several of them(including participating in both Wars, a fight against the god Solaris and even Dimitri's Enerjak), Y is a patient person, is slow to anger, and even has a sense of humor that seems to evolve depending on the times. He also shares the Alliance's views that the children are the most important, and is quickest to lose his temper when they are threatened.
Y has been involved in many events throughout the Four Worlds, commonly participating in them if his interference is necessary. Unlike other Mega Force members, he does not have a set territory, and has "patrolled" the Four Worlds and helping where needed. Currently, he resides at the Storming Base as a representative of his Team. The kids love him because he tells them stories of his experiences.
Y's full moveset has never been shown, but it was confirmed he knows Flamethrower, Steel Wing, Dragon Claw, and Dragon Tail.
As Mega Charizard Y, Y's Ability is Drought, which causes strong sunlight to appear for 5 turns.
Due to his massive size, Y has what can be equated to super strength, as he can easily snap a thick tree in half with a single swing of his tail.
Even though he is immortal and incredibly strong, Y still has Type weaknesses. He is weak to Water, Electric, and Rock-Type Pokemon.