Adon
Adon struggles hard to get in against Retsu due to his slow approach and buttons. Secure a lifelead then turtle, and Adon can't do much. If you lose lifelead, the matchup becomes harder, but still winnable; try to keep a good distance from Adon and poke from afar.
Birdie
Eagle
This is a bit of an annoying matchup, because Eagle is capable of punishing your HK from most ranges. MK is your friend here, as it ducks under all but 1 of Eagle's moves. Limit HK to anti air or using it at the maximum range possible. Your damage is decent, so you can definitely contest him; he just has more opportunities.
Geki
This matchup might actually be quite frustrating if you're unsure how to play it. But in reality, it's very simple. LK is your friend. Make sure not to get hit by Geki roundstart. If you can close in on him, then he's already just about screwed. He may try to use LP or MP to hit you when close, but you can easily blow up these attempts by pressing LK. If he gets the lifelead, note that Retsu is basically incapable of mixing the direction up to try and mess Geki up, so while landing the first hit isn't hard for you, don't underestimate its importance.
Gen
Make sure you secure the lifelead if you want to win, because gen's defense can be a pain to get through. His MK/HK can counter a lot of your tools (anti airs jumpkick, ducks HK) but as long as you have a lifelead, it shouldn't be too hard.
Lee
Tough matchup for Retsu as Lee can blow up your defense with his HP. Don't try to turtle, and instead try to keep the Lee on their toes and counterpoke as often as you can. LP can knock them out of HP, but don't spam it, since Lee has other ways around it.
