Our general suggestion for the large family is make reference to them but be somewhat vague. Give us enough information to know where your character is coming from, but don't define it in too much detail so if others wanted to app the other characters they still have some freedom to work with it.

If he's similar to Mew, I'd suggest instead of making him a shifter, simply making him a Psychic Enlightened that has some similar abilities to the in-game version of Mew. Legendaries in this setting are exceptionally powerful and will likely possess abilities above and beyond what they do in canon, so if you stick closer to the game mechanics and tone down some of the more overpowered things you should be ok.