Person who actually likes PoFV checking in.
Top Tier (against AI):
Medicine: You should not see a match last more than 90 seconds. The pixel-perfect dodging the AI uses doesn't work when they're slowed down.
Aya: Pixel-perfect dodging also doesn't work when there's a giant red thing coming at you at high speed. Not recommended because very fast bullets will be reflected at you.
High Tier (against AI):
Reimu: Large yin-yang orbs and the ability to trap foes with homing spells. Does not score very well. Occasionally her spells will send fast bullets toward you.
Sikieiki: Godly scope and an even godlier Lv. 2 for scoring. Main offense is the Last Judgment laser, which hits a fair amount of the time.
Mid Tier:
Marisa: The standard for playable characters. Good scope and the charge time buff is very useful, but the chance that her lasers will hit the AI is pretty small (but at least it exists).
Cirno: If you're fine with a manic focused speed and a scope that only allows you to body-check the butterflies; she can punish the AI with her icicles and play a great defensive game with spell spam (and still score reasonably well!).
Sakuya: The only thing worse than playing as Sakuya is playing against Sakuya. Her EX attacks slay the AI at the expense of an awkward scope and an awkward time stop.
Low Tier:
Yuka: A slow character that winds up having to dodge fast bullets. What's wrong with this picture? She's sort of okay at hitting the AI because her flowers are huge, but she's difficult to use once the bullets start flying.
Tewi: Her autobomb isn't worth basically having to play unfocused. Her EX attack surprisingly does well against the AI.
Bottom Tier:
Lyrica: See Reisen, but with a better scope.
Reisen: Lv. 2 spam will do nicely for scoring, but she has no attacks that will hit the AI... leading to a contradiction.
Komachi: Has no attacks that will hit the AI. With that in mind, ranges from okay to terrible based on how you abuse her scope.
Mystia: I don't get what's so great about her scope. Moves very quickly and has no attacks that will hit the AI.
Youmu: Her Lv. 1 shot is a rip-off of epic proportions, she doesn't score well, and while her blue knife spam has a greater-than-zero chance of hitting the AI it's not worth it on your side of the screen.
Something that I didn't factor in because I don't remember who fights whom:
*Yuka is easier than Komachi. I'm not sure if Reimu is easier than Komachi.
*Aya is much easier than Medicine.
*Reimu is easier than Sikieiki.
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You can't change to your alt costume during Story Mode.
I think in Match Mode you need to hold Shift as you select your character.
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Reisen's Lv. 1 attack would be much better if it had any chance of reaching the top of the screen and hitting something (considering it's slow-moving and usually detonates when it hits a spirit that's about a centimeter in front of you).