Which skills are best to level up for who?
I know Patchy is good with MAG.
Meiling is good with HP and DEF(and MND).
Should I level up Che'n's speed? Or should I level up her ATK?
Character - Personal Preference - General Consensus
* indicates I switched builds halfway through the game
Reimu - Magic/Mind/HP hybrid - Defensive build
Marisa - Pure Magic - Pure Magic
Remilia - Hybrid Attack - Pure Attack or Pure Defensive
Patchouli - Magic/Mind Hybrid (offensive slant) - Pure Magic or Pure Mind or Hybrid
Sakuya - Mixed*(Attack -> Defensive -> Attack, nearly unusable) - Pure Defensive or Pure Attack (are actually good)
Chen - Attack - Attack
Meiling - HP/Defense heavy hybrid with Mind- Pure Defense, Heavy Defense/HP hybrid
Cirno - Attack - Attack
Minoriko - Magic/Mind Hybrid* (Magic -> Mind) (heavy defensive slant) - pure Mind or very heavy Mind/Magic
Youmu - Attack/Defense hybrid - Pure Attack
Rumia - Magic/Mind hybrid (offensive slant) - Pure Magic
Alice - Magic/Mind hybrid (offensive slant) - Pure Magic
Yuugi - Attack/Defense hybrid (offensive slant) - Pure Attack or Pure Defense (does anyone use her that way, anyways)
Wriggle - Mixed* (Attack -> HP heavy defensive) - Pure Defensive (Balanced or else whatever tank your team needs)
Aya - Attack - Attack
Iku - Magic/Mind hybrid (Magic -> Mind) (heavy defensive slant) - Pure Mind
Komachi - Attack/HP hybrid - depends on what you need, but Attack and HP are the only stats she'll need
Sanae - Magic/Defense/HP/Mind - Defenses/HP/Speed or Pure Magic
Suwako - Attack - Attack
Nitori - Attack - Attack (gamebreakingly good 2.06+, mediocre 2.04-)
Ran - HP/Mind/Defense hybrid - Either Pure Magic or Defensive
Reisen - Magic - Magic
Eirin - High HP Defensive - High Mind/Defense defensive
Tenshi - Mind/Defense heavy hybrid with HP - Mind/Defense split, enough HP to survive Rasetsu Fists (this can come from gear or SP though)
Mokou - Defensive attacker (heavy magic with HP/Mind/Defense all balanced) - Pure Magic or Defensive Attacker
Flan - Attack - Attack
Orin - Attack/Mind hybrid - Pure Attack
Suika - Attack/Mind hybrid - Pure Attack
Yuyuko - Magic Defensive Mage* (Magic -> Mind -> Magic) (unusable outside of DTH) - Pure Magic (is actually good)
Kaguya - Magic/Mind Hybrid - Pure Magic
Yukari - HP/Defense/Mind - Defensive (whatever your team needs)
Kourin - Balanced - Whatever your team needs (Pure Attack, Pure Defensive, tank with hitting power, attacker with staying power, etc.)
Maribel - Magic - Magic
Renko - Mind heavy tank - Pure Speed or Magic Tank
Okuu - Magic - Magic
Kanako - Bulky Mage (see Mokou, except with good magic) - Pure Magic or Bulky Mage
Yuka - Magic - Pure Magic or Bulky Mage
Mystia - Attack - Attack
Keine - Balanced - Probably Defensive
Shiki - Attack - Attack
notes:
Reimu does well with any stat, considering I have thrown 40 levels into SP and she still has excellent stats. Don't do it though. Sakuya's pretty useless as a tank if you do a lot of switching, but is a great stay-in attacker if you build her for it. If you don't switch, Sakuya's a godsend. Aya and Mystia are pretty freaking bulky. Minoriko is probably the third/fourth best character at shrugging off magic attacks after pure Mind Patchy and pure Mind Iku, and possibly pure Mind or heavy Mind Yukari.
Yuka is NOT outclassed by Marisa, with a multi-target Master Spark and the ability to do minimal support while attacking. Maribel and Reisen's otherwise mediocre stats are made awesome by their buffs (same principle applies to lategame Remilia). Nitori is gamebreakingly good in version 2.06 or the Special Disk, and fairly mediocre otherwise.
Eirin is probably best off never touching HP with her level up points, considering she can overheal. Trying to use Sanae exactly as you would Minoriko leads to her getting heavily outclassed; play to her strongpoints - either build up her defenses and speed to throw Miracle Fruit out fast and heal status, or bump Magic to have a strong heal, but makes her fragile.
Rumia, Mokou, Yuyuko and Alice all have the potential to hit like a truck despite otherwise mediocre magic stats. Alice and Rumia level up fast. Alice's Return Inanimateness has a comparable formula to KOi3S, only losing out because Yuugi has massive attack power, while Alice's magic is merely solid. Mokou needs to hit weakness in order to really do well in terms of damage, but Fujiyama Volcano does well enough even without it. Yuyuko's Saigyouji Flawless Nirvanah is ridiculously strong due to its formula. Rumia's Moonlight Ray is awesome.
Kaguya is one out of two characters who can use a viable defensive stat ignoring move, and the only one during the main game. Shiki is gotten late in the Post Disk.
Keine and Rinnosuke have solid enough stats that if you equip the right gear, you can spread your points everywhere and still have them do well. Specializing them is probably better, though.
With the possible exception of Flan, who should be built pure Attack anyways, but can actually pull of a hybrid due to her ridiculous Attack power, always focus on a composite attacker's higher attacking stat.
Yuugi and Suika are almost interchangable. Suika is slightly weaker with better overall defenses, better defensive support options, and a Wind nuke while Yuugi is noticeably more physically bulky, a bit stronger, doesn't have a buff, but has a better overall offensive attacking set, and better offensive support. Either way though, most likely thing is you're going to buff and nuke.