PsychoLogical
Black and White, Yin and Yang, Light and Dark.
And yet unfortunately they’re just entirely shafted except for Umbreon who for some reason is the one that really got the push like any strongly marketable mon would.It's a shame too because Eevee's Evolutions are the exact reason why Eevee got so popular to begin with.
If it wasn't for them, Eevee itself would've faded into obscurity faster than Skitty