Characters: Ijyuin Akira and Tomoe Hotaru
Fandom(s): Akira is from 20 Mensou no Onegai/The Man of Many Faces and Clamp Campus Detectives; Hotaru is from Sailor Moon
Spoilers For: probably both series.
The Evidence: Firstly, both of them have very unusual family situations. They could get together and discuss the crazy things their family did the last week without having to feel weird about the fact that they have two mothers and father (in Akira's case) or three mothers (in Hotaru's).
Just as a little more detail, Akira lives with his two mothers, who are identical twins- no one knows except for them which one actually gave birth to Akira. His father, who only makes an appearance once a year (dressed as Santa so Akira doesn't know who it is) is the original thief 20 Masks. Akira's mothers taught him to be a master thief as well, and his mothers send him after things that they want (great things to be teaching your kid, right?) He also does all the cooking and cleaning for them (he's a masterful cook). Quite frankly, he acts older than his mothers. ~_~ Despite the fact that he's quite naive, he's very intelligent, attending the prestigious Clamp School. Math is his best subject, as is exhibited by the fact that he's the student council treasurer.
Hotaru, aka Sailor Saturn, has three mothers (though Haruka she calls "Haruka-papa"), all of whom are adoptive. They don't make her steal things, but with them she does fight evil.
In addition to having the insane families factor to commiscerate on, Akira and Hotaru are both horribly cute and sweet characters. Their personalities would get along very well, and Hotaru would probably be tremendously flattered every time Akira offered to cook for her (which would be all the time). They're both quite intelligent and would likely have a lot to talk about, intellectually.
Conclusions (a.k.a. Why They Should Meet):
(I suck at this...)
Akira and Hotaru have very compatible personalities, and they would have a lot to talk about. They're on the same intellectual level (though Akira is likely a lot more naive) and would enjoy each others' company greatly.