Marisa is one of the usual stray yukkuri. Her father died when hunting, and her mother died in conflict with other yukkuri. Her dying mother is ready to offer Please Eat Up, but got killed before she said it.
Then a human killed the Marisa who killed that Reimu because it's too noisy. This experience traumatized Marisa and made her avoid both yukkuri and human. She also follow her mother's last wish, to eat the carcass even though it smells really bad.
Later, Marisa who never know what else to eat became scavenger of yukkuri's carcass. She managed to grow into adult like that and staying out of other yukkuri's view.
One day, she met a Reimu and Ko-Marisa in condition like what she used to be in. But instead of feeling pity and adopt the little Marisa as her mother asked, she attacked Reimu, triggered by starvation and the knowledge that Reimu is not a threat. Here, she learned that dying yukkuri will smell bad.
But she also learned, that living Koyukkuri are delicious and not dangerous. Marisa switched her food from hunting carcass to hunting koyukkuri.
Troubled yukkuri in the park asked their leader Patchouli who suggested to keep adult with the Koyukkuri. Marisa is still too afraid of adult yukkuri, so she keep hiding until starvation made her attack a Ko-Alice who is alone.
Big mistake because that Alice is a pet and Marisa's action is recorded by surveillance camera.
After trapping Marisa with a Sukuna, the park manager throw Marisa to the park's yukkuri herd, telling them that Marisa is the one who made the Koyukkuri disappear, and that she ate them. The park's yukkuri can do whatever they want, just make sure Marisa is dead.
That way, he killed the Marisa who dare to attack a pet, and also let the yukkuri in the park relieve a bit of their stress.
