NOTE: This story follows the timeline used in Chuhou Joutai, and not ZUN’s actual timeline.
Mima’s early years
Mima was just any ordinary humanoid who grew up in Gensokyo during the Edo Period, frequently making trips to the Hakurei Shrine on a regular basis. Of course, this was during the years before Reimu Hakurei was even born. She also loved to explore the surroundings of the Hakurei Shrine, including the Youkai Mountain, the Human Village where she was born, and the Forest of Magic.
Mima is cursed
On a beautiful spring day, a terrible calamity from the island that houses the Scarlet Devil Mansion was ready to strike the Human Village. On order from her master, she sets out to the Scarlet Devil Mansion to assassinate the current headmaster. During that time, Remilia Scarlet wasn’t the mistress. When Mima arrived at the mansion, she was met with great misfortune. The doors went on to be permanently sealed, preventing Mima from escaping.
The headmaster offered her two choices to leave: either marry the headmaster (who was a man and not Remilia) or be cursed permanently. Mima declined the marriage proposal, and the consequences resulted. She had her clothes with the exception of a swimsuit stripped and from a spell the headmaster placed on her, her legs fused into a tail, causing her to resemble more of a mermaid.
Mima sets out on her new life
Mima was flown out of the Scarlet Devil Mansion and into the misty sea. With Mima stranded on the coast, she was plagued with frustration and depression. Over the summer, Mima’s only source of survival was eating seaweed and fish, though she had to be very careful not to get mercury poisoning. As winter was approaching, she began to shiver daily, until someone showed up.
Mima becomes humanoid again
Mima’s eyes caught a brownish blonde-haired man wearing a kimono. As she looked at him, she asks, “Hey you, have you heard of the Scarlet Devil Mansion?”
“Sorry, what?”, the man replied. Mima proceeded to ask the man for comfort from the cold. The man introduced himself as Kirisame-sama, a humanoid from the Human Village. Realizing Mima had been hit with a curse that caused her mermaid transformation, he took Mima into his own home for shelter, wrapping her in a kimono for warmth. Since Mima was still a mermaid, she could not walk, having to use a wheelchair to get around like a disabled person.
Eventually, the couple’s relationship began to grow, and once true love’s first kiss came, something remarkable happened. Following the kiss, Mima’s tail began to dry up, something that shocked them both. The tail began to crack after drying and began to fall apart, revealing Mima’s legs again. She realized she was humanoid again after almost a year of being with the curse.
Both Kirisame-sama and Mima secretly married, and Mima gave birth to her one and only daughter Marisa, who would later go on to become the founder of a monarchy within Gensokyo (that monarchy would become the Federal Kingdom of Eastern Kozan in the Chuhou Joutai world). When Marisa was two, Mima decided to wish farewell to Marisa as she went out to slay the Scarlet Devil who cursed her in the first place.
Mima returned to the Scarlet Devil Mansion, but this time for revenge. As Mima made the initial attack with a sword, the headmaster retaliated by throwing a kunai through her heart. Mima died of her wound almost immediately upon impact, and her soul ended up what she is now. As a ghost, Mima regularly made visits on Marisa and her father to check on how they were doing.
Upon her death, her soul went into the Netherworld like usual, although she was still allowed to return to the overworld at any time. Eventually, a Shinto shrine dedicated to her was built in the city of Kanjotaki in New Saitama province in Eastern Kozan.