Adon
This matchup is mostly dependent on who has the lifelead. Though the matchup is arguably more straightforward for Retsu, it's not necessarily easier. Pay close attention to the health in this matchup, as it usually determines who has to take the offensive.
If you have lifelead, you'll have the advantage. HK is what you'll be using for most of your defensive gameplan. It outprioritizes and outranges all of Adon's moves, so you'll usually be able to safely keep him at bay and widen the life gap further using it. Beware of Adon getting close to you, as his options up close are much better than yours and do massive damage. Also keep in mind that Adon may be able to take advantage of trades to take the lifelead, so always stay at the range where his attacks can't reach you.
When forced to go on the offense, the matchup becomes tougher, but not hopeless. While jumpkick can be useful for approach, you have to pick the right time to use it very carefully due to Adon's MP/HP being able to easily anti air it. Backing Adon into a corner and poking with HK can be a good method of offense, and if he tries to regain space, that's the best time to jumpkick if they aren't expecting it. Take advantage of your superior range.
Adon's jumpkick can be anti aired with HK or an earlier jumpkick of your own. Though generally your moves will take priority over jumpkick, be careful of it due to the high damage it deals. At the very least, it'll deal less damage if it hits air-to-air. A niche but potentially useful thing to know is that MK can serve as a counterattack to Adon's MK/HK if he's pressing it at close range.
Keep a good distance from Adon and maintain the lifelead if possible. HK is your best friend. Patience is extremely important in this matchup.
Video of the matchup: https://www.youtube.com/watch?v=ec1EEFfXG2Q
Birdie
This matchup is just miserable. Birdie is unable to be launched while he is attacking with his headbutt, and as a result, even Retsu's best headbutt answers, which are unfortunately also his slowest tools (MK, HK, Jump Attack), leave Retsu very disadvantaged on hit, allowing Birdie to just headbutt again and score a lot more damage. Much like other Birdie matchups, once he is in, there is very little Retsu can do to get him away. Simply put, Retsu lacks a sufficient response to headbutt, and so Birdie can bulldoze him with no real consequences.
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.
