I finally made a reference sheet for YingYang. YingYang will mainly go by they pronouns, but I will also use she pronouns for them.

Name: YingYang
Species: Espeon (Sun Pokemon)
Gender: Agender (they/she pronouns)
Moves: Psychic, Dazzling Gleam (Fusion Flare), Shadow Ball (Fusion Bolt), Hyper Beam (Draco Meteor)
Differences with Mega Evolution: YingYang's mega evolution is a lot different than their regular forme. Mega YingYang is Psychic/Dragon, and their stats are significantly higher, even their Attack stat, which is Espeon's worst stat. Mega YingYang's fur color and eye color are different. Also, their mega evolution has different moves, but they keep Psychic. Mega YingYang's only accessory is a Honedge scarf.
Biography: YingYang is the leader of the trustworthy Team PokeWhite and popular for their strength. Almost everyone who knows them idolize her and thinks she is very powerful. They did have a weird past, but most of those memories are forgotten. YingYang has a mega evolution.

NOTE: I am aware that Espeon does not have an official mega evolution. I am just using my imagination.