Gen is the trickest character to master in Street Fighter IV, by a long way.
That's because you have to learn two characters - Gen in his default Mantis stance (PPP) and in his Crane stance too (KKK), where EVERY move changes.
Therefore, you have to learn how to use his Mantis stance (which is more defensive) and his Crane stance (which has better combo potential) and sometimes, switch between the two mid-combo. There's a lot to learn, so no time like the present...
Hayakurenko (mash P - Mantis)
This move is similar to Honda's Hundred Hand Slap but it has the added advantage of smashing through Focus Attacks, so it's important for that reason alone. Safe when blocked and with decent damage, it's only real drawback is its short range.
Gekiro (back, down, down/back + K - Mantis)
Gen's anti-air move can be a little inconsistent. It puts him in a good position when it connects though, allowing you to set up a Hayakurenko or throw. You can tap kick once it connects for extra hits and damage.
Jyasen (back, towards + P - Crane)
A roll move, similar to Vega's rolling claw attack. Pretty good for chip damage and it can be Focus Cancelled for further mix-ups. Stick to the LP version for a quick and long-reaching poke.
Oga (down, up + K - Crane)
This is Gen's mix-up move, as he leaps to the wall and then comes back down in various different ways, depending on what button you press to follow it up with. You can use this offensively, where you use the unpredictability of the wall moves to score hits, or defensively. Just cancel any move into Oga and Gen will fly to the wall, taking him out of danger.
Zen'ei (quarter-circle towards, quarter-circle towards + P)
Gen lowers his head and charges towards his opponent. Good for ending combos and punishing whiffed moves from distance and can sometimes catch players jumping as well.
Zetsuei (quarter-circle towards, quarter-circle towards + PPP)
A more powerful version of Zen'ei. If you connect with Zen'ei, you can follow up with this Ultra.
Ryukoha (quarter-circle towards, quarter-circle towards + KKK)
One of the coolest looking Ultras in SFIV, Gen leaps into the air and catches his opponent before spinning them with a tornado to drain their energy. You can use this to grab anyone trying to jump at you. The threat of it should be enough to deter players jumping, which means you can concentrate on fighting them on the ground.
Basic Combo: Jump in HP > MP xx Hyakurenko
Advanced Combo: (Crane) cr.LK > Mantis > Zen'ei > Zetsuei