Kangustlocke! (the diary)

[EeveeBailey]

I agree with @Nekra. You'll need to have a Nuclear-type if you want (and are lucky enough to get an Eevee) to have a Nucleon on your party. Plus, there's no need to forfeit a capture -- even if you can't actually use it at this stage in the game (other than to get Nucleon), it may come in handy in the future, if just for having something handy to trade.