It's quite broken.
This game has quite a few broken characters, really:
Sanae + Iku/Okuu: Heal, reflect, resurrect, repeat.
Yuuka: Self buff for 40% damage and evasion boost, double spark for 300% damage with 20+ crit... Heck, after 3 reforms, my Yuuka has 950 attack power, charge first round, have Suwako buff the team hit rate, Yuuka can wipe S rank battles on second turn, and EX rank battles on second turn with the double damage ENV sphere. If she buffs self, then it takes her 4 turns to fire off her spark without ENV boost(3 with Okuu or Marisa restoring her SP), but she instantly kills all residents of Gensokyo aside from high level reform battles if she manages a crit. Without Yuuka I wouldn't have a complete collection of perverted fanservice avatars.
Aya: Nitori without heal, but if you load her up with 6 evasion spell cards...?
Suwako: 91 starting MP, and huge regen at that. She basically never needs to charge, just keep firing off her buffs non-stop with resets inbetween.
Komachi: A bit weak with starting MP, but her last spell card is a rare 300+% mid-hitter that can instant kill if you feel like powerleveling her by stuffing her with nothing but accuracy for reform runs, plus she has a skill to shield your entire team if it comes down to defense.
Yakumo family: They're all mediocre by themselves, but due to the ease and power of their Union card, you can easily have Chen and Yukari get ready for Ran to fire off a card that hits for 400% at both close and mid range on first turn, with near 100% accuracy. After reform, Chen with a high MP related stats can boost Ran with +50% attack power as well, while Yukari boost Ran with accuracy. Only problem is Yukari is exceedingly slow compared to Ran and especially Chen, so she needs at least a 6 speed boost over Ran to go before Ran. This can be done with equips, or with reforms.
Youmu: Strongest close range attack that rivals some union spellcards in damage. Too bad her MP is horrible... Plus she is one of the people who possess a damage boosting debuff.
Kanako: She has an all-range attack that boosts the damage of the next attack that connects. Combined with strong solo attackers like Komachi, Yuuka and Youmu...
You get "assist points" for completing dream events and leveling up spell cards. You use those points to level up assists.
The assist abilities are easier to lump into groups than to do it by character, since there's repeats with just different names.
Increase Accuracy vs Close Range:
Increase Damage vs Close Range:
Increase Accuracy vs Mid Range:
Increase Damage vs Mid Range:
Increase Accuracy vs Long Range:
Increase Damage vs Long Range:
Recover HP per turn:
Recover MP per turn:
Increase Critical Rate:
Increase Critical Damage:
Recover MP when hit:
Recover MP when dodging:
Decrease damage taken:
Increase Love Gained:
Decrease MP required for spellcards:
Each gift varies in magnitude for each character. If you give a gift that really suits someone, coupled with the 2x multiplier from quiz battles, it's quite simple to get the required amount of affection needed to use any avatar in 2 or 3 gifts.
The gifts are in order of:
Ball of Yarn
High Grade Red Tea
Traditional Hair Ornament
Gold Coin(Reimu really loves this one...)
Hand Knit Scarf(nevermind the fact that you probably ripped it off of some Sin Bag and didn't knit it yourself)
Jewel(Reimu doesn't like this one for some reason, probably not very liquid)
Beautifully Packaged Magic Book
???(didn't get this one yet)
Scroll of Hyakki Yako
About reforms: During reforms, you can increase stats you normally can't. This is very nice for certain characters without good starting MP but has stuff you want to use right off the bat(Iku) or high hitters who need to charge too long to use their better attacks(Sakuya and Youmu), as well as slow characters who never seems to get a move on with their turn(Yukari and Satori). But it's not much use to characters like Suwako who has 95 starting SP when you get her out of the machine. You can also dedicate entire runs for nothing but reforms, since you can stock up on 9,999,999 Love, reform, dump everything into 1 stat, reform specifically for that single stat, and level up again. This way you can build extremely powerful characters very quickly(lol Yuuka with 600 attack at level 1 on 2 reforms). S/L as needed.