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.
Even though this matchup is miserable for Retsu, there are certain strategies that could give you the victory against the counterpick or if Birdie is not playing at his full potential.
F.JA is +15f when hitting Birdie while he is in his standing animation, allowing you to perform a back walkdash and execute an HK, which will always win against Birdie's LP. Now, if Birdie doesn't know he can perform an LP or if he wasn't paying attention to the play, you can perform a forward walkdash and execute an MP, wait a moment, and then perform an F.JA to start the mixup again.
Use MK sparingly, as it is -5f on hit and Birdie will always have the advantage in this exchange.
When Birdie performs his Forward Jump Attack (F.JA), the best option is to use LK instead of HK. There are two reasons for this: HK allows Birdie to hit you twice if you miss, and LK gives you a +7f advantage. You can also use LP, which gives you a +12f advantage, but this will change your number of hits to finish the match (whether this is good or bad depends on the player and the context).
Eagle
While this may seem like a daunting matchup, since Eagle is capable of punishing your HK from most ranges, it is very possible with proper patience and defense. HK is safe at absolute max range, so the key to this matchup is being able to space properly in order to safely do damage and keep away from Eagle. Though Eagle's individual moves overall do more damage, you can match him in the damage race with HK and jumpkick.
If Eagle manages to get close to you, things might get rough, but you're not screwed. LP does beat all of Eagle's moves by one frame, and you can utilize MK to duck under any Eagle move excepting his kick. Chains like LP -> MK -> HK or just MK -> HK are viable ways to get Eagle away and turn the tide in your favor once again.
The video below is a great dense and informative guide to the Eagle:Retsu matchup. It is in Spanish, however, so unless you know the language you may need to use Youtube's auto-generated captions. https://www.youtube.com/watch?v=YF6FsMVm2Gc
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
One of those matchups that is heavily centered on getting the lifelead. When you have the lead, there isn't too much to worry about other than Gen's quick jump attack. If Gen takes the lead, however, things will become difficult. Thanks to Gen's MK/HK, Retsu will have a difficult time approaching and winning back the lifelead.
Should you lose the lifelead, there's no need to panic. Recklessly rushing toward or jumpkicking Gen may just lead to getting clipped by yet another kick and the life gap widening even further. It's best to attempt to punish the recovery of his MK/HK with your own HK. This is admittedly quite difficult due to the short window where Gen is vulnerable, though. Utilizing HK will also allow you to successfully deal with any attempts for Gen to surprise you with a quick jumpkick or walk-up MK/HK.
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.
Joe
Neutral, and specifically round start, may be a challenge against Joe. If you're up-close against him, it's a gamble whether you come out unscathed or not. MK is a good move to go under his high hitting normals, but his gut punch is a strong move in the matchup. Gut Punch and his MK are good anti-air normals, so don't use Forward Jump Attacks often (unless you know what you're doing). Joe's Forward Jump Attack is very fast and is a valid opener versus Retsu, but he does have a few preemptive tools against it. Neutral Jump Attack beats it clean, and both MP and HK can catch Joe while he's jumping forward. Your win condition is to force Joe to the corner because he can't do anything against HK spam once his back is to the corner.
Mike
Surprisingly one of Retsu's most brutal matchups. There's not much you can do against Mike, as he can kneedash under any move of Retsu's other than MK, and MK is too slow and weak in this case to match the might of Mike's kicks. It is generally advised you pick a different character for this one.
Retsu
Sagat
You won't survive
Anyways, you can theoretically loop him if you can get him to back in to corner, but no Sagat should fear you. Otherwise, he's stronger than you, faster than you, zones better than you, and doesn't get launched by any of your moves. Finally, as you can't loop him out of corner, he can often throw out tiger shot carelessly, and if he hits, it's likely round over.
