>>50099456
Fact: She could not settle in the village and left for the woods in order to live with her own people.
Fact: She has an unnatural hair color that could not have come from the proud sons and daughters of Yamato.
Fact: She has a notable ability to use magic despite a complete lack of talent.
Fact: She is incredibly reckless with a near-total disregard for her own safety.
Fact: She has an immediate rapport with any fairies she meets and is quick to join in on their antics.
Fact: She "survives" clearly lethal injuries despite not possessing a spiritually or magically enhanced body.
Fact: She compulsively engages in sneak-thievery.
Fact: She is simply far too short to be considered truly human.

All this points towards Marisa being at least half-fairy, and that she's the result of Mister Kirisame nutting in Luna Child's womb one faithful moonlit night, who then dumped the baby on him after it was born and promptly forgot about ever giving birth, never recognizing her own daughter.
It's tragic, truly.