Strategies
Sirou is a fairly versatile character, and might want to play very differently depending on the match up. A constant is his defense, as he can be an absolute wall against almost every character.
Neutral
Sirou has a lot of strong tools, but his rock is C.HP. This button shuts down entire match ups with its fantastic range, great frame data, and generous hit box. When in doubt, this move beats a lot of strong options while being very safe, since it recovers fast enough for Sirou to anti air most jump attempts. Use C.HP to feel out your opponent and define the pace of the match. When anti airing, Sirou is in a class of his own with S.HP. This move is genuinely ignorant as hell, with a hitbox that covers above, in front of, and BEHIND Sirou. The range is slightly outclassed by Dolf's C.HP, but S.HP has no blind spots, meaning it's almost impossible for an opponent to sneak in a jump in versus Sirou. Other, more situational but still strong tools include NJ.LP, which serves as a persistent hitbox but is fairly risky and better served against a cornered opponent. 236HP is very good in certain match ups, since being a projectile, it can cover a lot of ground space quickly. Its 60 total frames make for a somewhat risky move, though, so it benefits from being used sparingly alongside less committal tools.
Offense
Sirou doesn't always want or need to go in, but when he does, he has a few moves that can be useful in opening his opponent up, even without any lows. The most simple is S.LP. This move has a lot less range than C.HP, but is +2 on block, so can be used to push your opponent to the corner while being advantageous. This is important, since Sirou's goal in midscreen pressure isn't necessarily to get a hit, more likely you're trying to gain screen control, since Sirou basically plays himself once your opponent's cornered. Of course, you may want to sneak in a throw to end the game faster, and an excellent tool for doing that is J.HP. J.HP is a great cross up, but is also vertically massive. This makes it hard or impossible for a lot of characters to anti air a high J.HP. This gives Sirou a mix up, as high J.HP is 0 on block on most characters, so Sirou wants his opponent to anti air and get hit, but if they just block, expecting an early J.HP, Sirou can delay it to be plus for a throw. The strength of this mix up varies from character to character, but in certain match ups it can be devasting.
Corner Pressure
Corner pressure deserves it own section with Sirou because of how drastically his goals and common rewards change with good screen positioning. In the corner, Sirou either wants to get a block infinite or a throw. This is fairly easy for him to do because of two moves: S.LP and NJ.LP. S.LP is the less powerful of the two, but it is still important. S.LP makes you +2 with no pushback in the corner, which means making your opponent block one S.LP with a life lead basically always means the game for you. This is because NJ.LP is almost impossible to escape in the corner and is an actual block infinite on tall characters. Simply hit S.LP, then NJ.LP, and repeat until the timer runs out. Alternatively, you can try to sneak a throw in a any point, which is almost always what you want to do if you aren't confident in your execution or don't have the life lead.
Defense
Defense is when Sirou goes from being strong to being annoying. Sirou's defensive game is boosted by a few key factors: his fast throw wakeup speed, his quick buttons and anti air, and his projectile: 236HP. The first two traits are useful, the last is disgustingly broken. Sirou's Swordflame is a grounded projectile, one of the only two in the game, and that gives it one important quality: it effectively has armour. If Sirou is hit out of the start up of his Swordflame, he will play out his regular hit animation, but the projectile will still come out. Swordflame has 18 frames of startup, meaning there is often no way to avoid it if you hit Sirou during the move. With a slower projectile, you might be able to move out of the way more often, but against Sirou, you block or get hit, and with a grab game as good as Sirou's, you probably don't want to block. This is what truly tips Sirou over the edge: he might be strong in neutral, and have serviceable offense, but the worst part is that even if you manage to get in and pressure him, he has debatably the best defensive tool in the entire game. Being immune to combos and most throw loops is nice, but that is truly complimented by Swordflame, since you have no choice but to pressure Sirou, and when you do, it won't last long. Swordflame can sometimes be baited with a jump, but you should be using this move all the time, as it basically removes consequence from 80% of interactions.