Her given task is guarding the Ishlith Shrine, but she fell asleep, probably due to boredom. When the player comes around and obtains the Fish Key, she wakes up and becomes rather furious upon realising that she kind of failed her task.
Her attack pattern in the actual boss battle is very beginner-friendly, making her a fitting tutorial boss. She always starts with stomping her feet on the ground in a fit of rage and afterwards attacks the player with her bunny spear. She only uses one combo which consists of three strikes. Upon finishing the combo, she starts stomping again and therefore repeating the cycle.
To defeat her, you simply have to keep some distance between her and you. When she starts walking, just jump over her, as she will not track your character, but instead goes to the position you stood in when she was busy stomping the ground. When she reaches your former position, she will start her combo; she cannot turn while doing that, so run up to her and punch Prim until she starts stomping again. Rinse and repeat.