Monster Tinder

[Amaterasu]

Mario

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Matango Dryad!

Matangos are a type of dryad who take the shape of anthropomorphic funghus. They are capable of rooting their legs into the ground and energizing themselves via photosynthesis, and more importantly they release spores from their "hats" which have various properties, such as causing hallucinations.
What does this all have to do with Mario? Well, as we all know our favorite plumber's normal girlfriend keeps getting abducted by Bowser, stuck behind all sorts of fortifications and bosses for Mario to then battle his way through, traversing various worlds from dungeons to volcanoes to snowed plains and green grasslands. And for some reason he still invites Bowser to go Go-Karting or to play football and tennis... and things never really stick with Princess Peach either.
Here's a wild theory: what if Mario has already met Matango, and is under the influence of her spores? The hallucinations may differ for whoever breathes them in, but there has been a case where an entire household thought themselves to be in a dungeon - with a shopkeeper, "random encounter" monsters, a wise oracle and everything. The spores may also change their effects over time, making the person see different visions of situations. Do you see the parallels?

Okay, that theory may be a little far-fetched, but let's take things from a less crazy standpoint. Mario is probably not happy about Bowser taking Princess Peach and trying to force her to marry him or be a mum for his (adopted?) children with surprising tenacity. He is probably not too happy about being effectively friendzoned by Peach and getting little more than a "Thank you!" and/or cake for traversing half the world to rescue her. But at the same time, he's not one to settle down and stop the adventuring. If he does he'll start lusting for adventure and entertainment again before long.
Matango can provide him with a safe way of living out his dreams, through her spores. She could portray herself as the princess that he must rescue, and place him in a "dangerous" world full of obstacles that he can overcome. There is no real risk to this because it's a hallucination at the end of the day, and if he left the "boundaries" of the area (such as by exiting the house/palace) the spores would disperse and he would see clearly again. Matango also has little risk of being kidnapped by Bowser as he would be affected by her spores - and she releases more when she feels threatened - plus she could root herself into the ground to at least make taking her away harder. Bowser may be a firebreathing monster but he would not rip a girl's legs off in a kidnapping.
Finally, Mario has a number of friends who are used to supernatural occurences, and Matango's spores don't make it easy for her to make friends. Mario could introduce her to the people he knows, which she would appreciate greatly! And I don't doubt that she'll take friendzone-Peach's place in Mario's heart in no time.