Erm, well...Her source material is projectile barf and uh, more egregious projectile barf (there's a reason why they call it bullet heck), so she would be a projectile zoner.
There are a few mechanics that they could use beyond that, though how they'd be implemented is a big question mark. So the first thing is that in Touhou, your hitbox is smaller than the character itself. It's typically a glowey dot in the center of their bodies that you can only see when you focus your fire. The "focus fire" mechanic is essentially the ability to hold down a button to change how your shots come out. IIRC, Reimu's default shot is homing, but when you focus it, it shoots straight forward, so you'll land more hits on a single target quicker, but it's harder to land your attacks. It's also worth noting that when you focus your fire, you slow way down, which is also helpful since it helps you microadjust your way through a wall of projectiles (along with the fact that you can also see your hitbox).
The most direct way that I could see these mechanics being implemented is that Reimu would have a normal hitbox, and would have a Neutral Special or something that would be a rapid fire projectile kind of like Fox's Laser, but a bit more mobile. She could then focus the attack, which changes the use case for the projectiles, but locks you in place more or less. While focusing, the glowey dot appears, causing her to take less damage from any attacks that don't hit this dot, but more damage from attacks that do. The focused version of the attack could also be a separate move.
Then there's the bombs, which originally cleared the screen of all projectiles, and did damage (though I think later games do add different types of bombs that act very differently. Reimu always has access to the standard one though). Bombs can also be used when you're hit so that you don't die, but you do have to trigger it on reaction. They also have limited uses, and are about as valuable as lives themselves. In Super Smash Bros., this would likely be an X number of uses only move like Wonder Wing that acts like a burst in other fighting games, though perhaps more deadly. I...wouldn't make the on-hit reaction version of the move very powerful though; just make it a "get off me" tool with little to no damage, or perhaps just make it a counter or give bombs super armour or something (they already destroy all projectiles and would likely be super punishable so why not?).
There's also a grazing mechanic that gives you more score for almost getting hit by things (score gives you bombs and lives), but that could be pretty strange to add. If it was, I'd imagine it would be something like rewarding the character for staying just out of their opponent's effective range.
As far as Reimu specific things go, she usually has an attack involving bouncing yin-yang projectiles (they fly straight, but bounce off of terrain), and if they wanted to reference the first game, they could make it so that Reimu can hit the projectile with other attacks to alter its trajectory. As a boss (which IIRC, only happens once), things get crazier. The most reasonable attack involves projectiles that fly straight in the player's direction, pause, then go straight at the player again. They do not lose their hitbox as they pause. Most of the rest of it is the standard projectile barf fair (though you could have attacks inspired by the pattern albeit
super committal ones), but there are two very interesting Spell Cards that she uses in the fights. The Dream Sign "Duplex Barrier" and Boundry "Duplex Danmaku Field" Spell Cards create two electric boxes; one inside of the other. They don't do damage to you, but it warps her projectiles. The projectiles will start from the inside of the two boxes, and move out. When they hit the first box, they teleport to the second one, and move into the center. Then when they hit the first box again, they teleport back to the second box and move outward again. Something interesting could be done with that.
All in all, since Touhou is a series where you have to keep a level head against the mass of projectiles, I'd imagine Reimu would end up one of two ways:
- She's the player, and her preferred range is just out of her opponent's reach.
- She's the boss, and she likes to stand back and barf projectiles everywhere, with her opponents having to be the one to try and bob and weave through them to get their gameplan going.
She could easily be one of the more galaxy brained zoners if all of the projectiles they chose for her were on the weirder side, and would almost certainly end up being a pretty polarizing character since some characters just can't deal with hard zoning. She's also likely to be a featherweight since you die in one hit in the original series.
EDIT: And pretty floaty too since they're always flying around. I wouldn't be surprised if she had an emphasis on aerial play either.