So far... KoMarisa sincerely loves her parents so much. One day her daddy Marisa got killed in traffic accident, so she decided to bring home food for her mother. Apparently KoMarisa never realized her mother is a Deibu. From Reimu's words, seems that Marisa was once a pet, but she got thrown out for falling in love with Reimu. Ever since that day Marisa tried her best but not enough to satisfy Reimu.
Even after Reimu insult KoMarisa and her daddy, she still tried to ask her mother to take it easy together, which ends up with Marisa got tossed away and spend her days looking for Reimu.
One day, Marisa politely asks Mister Bug to share the food. When she saw a KoReimu, she wanted to share. That KoReimu claimed the food as hers, then starts calling KoMarisa as useless trash who shouldn't be alive and that KoMarisa is a poo poo born from a poo poo mother (before finally eating the leftover bread).
Marisa doesn't take this Reimu calling her mother a poo poo, so she fights back.
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Personal opinion: Marisa... would you marry the orphan Alice ?