General Strategy
Retsu is a defensive character revolving around keeping his distance and punishing opponent's mistakes.
For playing defense, HK is your best friend; great damage and super long reach can make it scary to approach. When playing offensively, stay out of the enemy's range, and look for any mistakes or mishaps they might pull, and attack.
If you can get your enemy into the corner, Retsu's corner pressure is surprisingly strong; you can alternate between using jab for jab pressure or walking out of your opponent's range and bullying them with HK.
Retsu's weakest spot is up close, but he is able to escape most situations with a jab, though actually getting back into your preferred range is difficult from there. Be careful against aggressive playstyles and don't let your guard down.
Retsu's forgiven more than other characters, but losing lifelead will still put you at a disadvantage, so be careful.