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She had been abandoned on the street with her twin sister, Molly, as newborn kittens, until Dr. Inagaki found them during his stroll with Momojirō and brought them home, because the two kids who were crying over them wouldn't leave because they felt sorry for them but weren't allowed to keep them, and Momojirō would not stop snuggling the kittens. Inagaki, however, tried to give them away as soon as he could due to his dislike of animals, but failed because even though there were some potential adopters, they all changed their mind at the last minute, despite Mikan's best effort to spread the word (by walking around town with balloons and posters tied to his tail). The Kusanagis were so concerned about this that they had to resort to adopting both of them for fear that Dr. Inagaki might want to abandon them again, only to learn that he decided to keep them in the end. She was given a red collar to help distinguish herself from Molly, although she grew up to be slimmer than her sister. She "married" Mikan (who, for some reason, didn't seem to ever age) and had kittens, one of whom was Ringo.
In the manga, her "marriage" with Mikan simply happened in secret, the result of which was discovered by Dr. Inagaki when he felt like her body was getting bigger, almost like Molly. In the anime, there was more drama to this: Dr. Inagaki strenuously rejected their relationship, to the point that they had to "elope" to an abandoned house. In the end, he could do nothing but accept it when Killy got pregnant.[source]https://en.wikipedia.org/wiki/Mikan_Enikki[/source]
