After playing this game for a while now and clearing the campaign, I can say the most broken characters are easily Rachel and Kate. Kate is pretty obvious; a skill that when you recruit her puts her at 75% Tension at the start of each round, which is insane for Server's Aces with Trick Serve, on top of Quick Slice which, when returning a shot, can basically let her get a Returner's Ace immediately after contacting the opponent's Serve by curving it to the same side she's on. It's actually absurd.
As for Rachel, Rachel just dominates with raw power. Being the highest natural power character, she just plain shoots at ridiculous speeds that rival top spins on most of the other team. She can get Returner's Aces by just smacking the ball on the opposite side of the opponent similar to Kate with Quick Slice, but with just a normal ass shot. The stun bonus on Power skills is just a neat extra.
Quick Counter is the best defensive in the game, because when you fight an opponent that uses quick moves, you'll regret not having it; Kate being the ultimate test on if you have it or not because she'll always start a serve with Quick Serve when you face her, giving you a fraction of a second to react if you don't have Quick Counter. Trick Counter seems hit or miss, but opponents love using Slices for some reason so I'd say that's the best other defensive to get if not Power Counter.
