Derp the Shinx.
it's so derpy, it doesn't even look intimidating. as such, it must have the ability rivalry.
have it defeat 15 pokemon of the opposite gender with physical moves to evolve every time
it may never hold an item, and it must lead your party throughout caves because it believes that its face is so awesome wild pokemon will be driven away (hint: it doesn't work but it never realises it)
also, it must sit out 2 gym leader battles that are not Volkner or Byron, and it must sit out 1 E4 battle that is not Bertha.