These Pokémon make up popular decks in TCG Pocket at the moment.
Pikachu EX: Relies on a bench full of other electric types to use; otherwise, it deals zero damage.
Mewtwo EX: The go-to psychic combo. Mewtwo destroys two turns worth of energy to use it's strongest attack, and gardevoir provides an extra turn's worth of energy for free. So gardevoir lets it spam it infinitely
Moltres EX: uses an ability to generate 0-3 mana based on coin flips to empower other pokemon in the back line. obviously, 3 tails means you get jack shit.
Dragonite: Very powerful and very fun, but the only current deck that requires splitting your energy type, which makes it a real crapshoot. with no reliable way to generate either energy, you're stuck with the luck of the draw.
Canonically, Youmu is like 30+ if memory serves me right, but I guess in Youmu-kun's canon she's like 14-15? Edit: Or well, I guess it would be possible for her to be as young as 12, since apparently a lot of people still count the years for this according to kazoe (long story short: it's an old age reckoning system by which babies are considered to be 1 by the time they're born, and a year is added to that on each New Year's day).
