The female protagonist from XY’s name is Serena…
… and the legendary Pokémon from X’s name is Xerneas.
Did anyone notice that XERNEAS's name, mixed up, is actually SERENA with an extra X?
S E R E N A
E R N E A S
and in the game series, Kingdom Hearts, a Nobody’s name is “an X plus an anagram of their original names from when they were humans.”
That could only mean that Xerneas must be Serena’s Nobody!!