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Spoilers!!!!!! Mima is Marisa from the future
Her magic grew so powerful she learned how to travel through time + becomes youkai
Great disaster hits Gensokyo, brought about by selfish humans
Marisa travels back to train Marisa in hopes of making her stronger than herself
(This is why her age is unknown and hates humans)
(Also many of Marisa's spellcards are technically her own)
(Also PC98 says Marisa changes her hair color, why not to green)
Marisa antagonizes Reimu to train her and tries to take the Yin Yang orbs because they are the secret to victory
When Marisa can't theoretically live any longer training herself she tells PC98 Alice her secret
Alice seals her soul and gives it to Yuka to look after
Yuka gets red eyes etc
TH13 Marisa takes control of Yuka to test if Gensokyo is ready for the disaster
!!!!!!