Koga

Koga is an insanely good Trainer card that unfortunately only works on two different cards. But, honestly, this card is what makes those cards good in the first place. Koga has two main uses in a deck: the first is to get an injured Pokemon out of the crosshairs without surrendering it and a point, and the second is to quickly and easily switch between two Pokemon that have synergy; in particular poisoning with Weezing, and then switching in to a powerful Pokemon card that's been waiting in the wings for its chance to shine. In that regard, Koga can be the make-or-break factor in pretty much any deck that can utilize his ninja skills.