Suika's backdash has a few invulnerable frames at the start. It also can be cancelled into specials, so you can reversal backdash and special a lot of meaty wakeups. Don't do it all the time, as some meaty moves (especially bullets) have really lengthy active windows and will hit you out of your special. Reversal backdash is pretty good in general though.
Suika is strong ground-to-ground and can actually footsie with Yukari. In fact, really good use of far 5a and charged 6a can make it really dangerous for anyone on the ground.
Antiair, Suika is better just backdashing (backrunning?) to deny the opponent an approach angle. 2b can be used as antiair with mixed success. It has exactly 1 active frame so you have to be pretty baller with it. Remember that you can cancel a backrun into a special such as OSB or firepunch to catch a jump attack. OSB is a pretty respectable antiair, all things considered.
On offense, your main string is 5a or 2a into 3a or 6a into 5b into highjump 9, then j6a at the lowest possible height. It's really crucial to get that minimum height j6a. If you whiff it, you're toast, so practice hard. This is probably the most technical thing about Suika. 5a 3a 5b hjc9 j6a 5b d6 j5a land repeat.
Suika's bullets are really dense, especially OSB. Don't throw lots of bullets, but do use 5b to provide cover and 6b or OSB to punish stuff. Fire Oni and C bullets are special-use, you only throw them when you think you can get away with it. White holes are a much better version of these bullets, they are faster so if you want more bullet pressure mixups you can definitely run white holes.
66c is your only graze attack and it gets its graze frames kind of late, plus it's really, really unsafe on block unless you cancel it into a rock throw super. Still you kind of need it.
OSB is mandatory in virtually all matchups. Some people run firepunch, but you basically need to MAX it for it to be really good. Only do that if you have a fast-drawing deck, eg. don't have 5-card supers or lots of reversals. I can see running firepunch in a deck that has Superdense Conflagration or something, but it isn't my thing.
vs. Reimu
Reimu sucks. She has much better control of the map than you do, and her pressure is as strong as yours. It's really a hard fight overall but there's nothing particularly special about it. If you can get into a melee ground zoning position, fight hard to keep it as it's your one strong position.
vs. Marisa
About the same as Reimu, except Marisa has less control and more damage. You control the screen a lot more reliably, since she has no decent graze but she has LAZERZ and you have no decent graze either.
vs. Sakuya
Fuck this.
vs. Alice
This matchup is so bad. It's not Sakuya bad or anything, but your sweetspot for melee is Alice's too. You can actually fight Alice in a footsie war if you can stay out of range, because the doll shotgun has a Dhalsim-style hitbox; the hitbox extends slightly forward of Alice's hand so you can actually hit it with your gourd. Don't rely on this trick because it relies on Alice doing the doll shotgun out of range. Also, if she does the air version it doesn't have the extended hitbox. Anyway, Alice is really strong at controlling the map, so you must absolutely keep her locked down. If she ever escapes and you end up blocking a jab or something, you're probably going to be dealing with doll bullet spam and crazy dumb mixups for a long time.
vs. Patchouli
Because Suika has no good graze attack, Patchy is strong here. Don't expect to walk away with this one. Also, Patchy's reversal specials are annoying. I'm not sure how Suika wins, this matchup feels impossible.
vs. Youmu
YES A GOOD MATCHUP. You are stronger on the ground than her, but always be careful as she's even faster than you are. Her bullet cover is good, but you have the best defense in the game -- backwards running. It's close to even, but Suika does amazing damage for less effort and has more zoning ability. There's nothing really weird about this match that I haven't explained somewhere else.
vs. Remi
Probably close to even, maybe a little Remi-favored. Remi likes to "jump in" using her forward dash, which is really annoying for you. Try to backdash to make her whiff, but it won't stop her from attacking. Consider backdash > OSB but remember, her dash grazes. You are much stronger if you can take to the air. j6a beats most of Remi's good options and Suika can actually fly with great mobility. j2a beats literally everything Remi has, but don't miss with it.
vs. Yuyuko
Actually not too bad. I think it's Yuyu-favored still, but it's not awful. It's not like she can reversal DP through your offense unless she likes pain. If she ever gets momentum, you can backwards run to negate a lot of the danger. Remember that you have babies, which block Yuyu C bullets, and that your B and OSB bullets go through her wisps. As Yuyuko, there is nothing special about this matchup, but you basically have to corner Suika in order to actually get the advantage unless she blocks something.
vs. Yukari
Like I said, melee zoning is a bit risky, but you can scare Yukari into thinking about it a little if you successfully land a counterhit f5a. The trick is to get her a bit afraid to do 66c all the time, so you can throw bullets on the ground more safely. Her C bullets actually dispel most of your dense bullets. Be careful. As Yukari, just play normally, except be aware of f5a because it does kinda force you to not spam 66c > supercancel and actually requires you to think on the ground.
vs. Reisen
This matchup is also really hard for Suika. See Reimu and Marisa, but harder. Her B bullets stuff you out of most approaches, her denser bullets eat yours, and her pressure is insane. At our level, it's close to even because Suika is rewarded more for actually scoring hits but Reisen is rewarded more for having anything blocked. The good news is that backwards run is still good if she does anything midscreen.
vs. Aya
No idea, probably like RAY-MOO
vs. Komachi
Suika favored, but nobody cares
vs. Iku
I actually think this is more viable than the JP players think? Suika is very strong once she gets OSB in this matchup. OSB is basically your answer to everything Iku does. Jump in? OSB. Attack on the ground? OSB. Throw bullets? OSB. Iku's still strong (she's Iku) but it seems very winnable for Suika.
vs. Tenshi
JP says it's even, I think it's Tenshi-favored. Nothing else to say really, again a lot like Reisen imo. Obviously she doesn't have as good a midscreen as Reisen, but she does tons more damage. Backwards run doesn't work very well here.
vs. Sanae
Any rushdown character can mess with Sanae spamming alt Kanako, so I think this is probably a roughly even or slightly Suika-favored matchup, since Suika can possibly take a round off Sanae before she even gets Kanako. If Kanako does hit the field, it's a bit rough. Sanae's 5c is rather dense, and she does have good bullet coverage, but you completely rule her world on the ground if you are allowed to get into f5a range. You also rule her face in the air.
vs. Cirno
uhh... f5a too good
vs. China
lolwut JP says this shit is even, Suika is like basically a better China. Ring kicks are quite powerful, so it isn't quite "get into f5a range and win" but China's offense is crappy and yours is airtight.
vs. Utsuho
Good damage, rushdown, good ground game, decent midscreen game. Yes, Utsuho is pretty outclassed here. It's definitely not an easy fight for her. Okuu basically has to run away in this fight, and she's like, the worst runaway character in the whole game.
vs. Suwako
Refer to the general Suwako post. Suika heavily advantaged here, mostly because Suika has incredibly powerful offense like Suwako along with a better midscreen and huge damage off jumping normals.