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Ultra Sun and Ultra Moon are an alternate retelling of the events of Pokemon Sun and Moon. Some might refer to this as an “alternate universe”, rather than insist the events of Sun and Moon did not happen at all. The story is “quite different” and more like a B2/W2 (despite not being a sequel) than a Pokemon Crystal/Emerald or Platinum version.
The Alola region is still comprised of 4 main islands, however the landmarks of some of the islands are in different conditions than they were before. One example I was given is that the ruins where Hapu’s house is located is no longer ruins and instead a full fledged town, that stretches larger south than the ruins you find in Sun and Moon. There are other larger changes, while many of them are small visual differences from Sun and Moon.
Ash Greninja’s “Battle Bond” ability is not exclusive to Greninja anymore, there are other notable Pokemon you will find in game that have this ability and look similar to their trainers. Lillie’s Alolan Ninetales is one of these Pokemon. This ability will be explored more thoroughly throughout the game, and only notable characters possess Pokemon with the Battle Bond ability.
There are no new Mega Evolutions, but there are more Alola Forms, mainly focusing on Gen 2. Some of these Pokemon include Gliscor, Bellossom, Ariados, Yanma, Sudowoodo and Quagsire, I’m sure there are more than what I was told. The purpose of Alola Forms of Johto Pokemon are to coincide with the Gold and Silver Virtual Console releases. There is a new final evolution for each Alola starter and requires a different method than normally, this was not fully explained to me.