The suffix "Gozen" was used for a wife of a Bushi (member of the warrior class) , such as Shizuka-Gozen. Yeah, I know what you're thinking: If Tomoe Ame marries Usagi, she'll change her name to Tomoe Gozen. It's a possibility...

Anyway, "-in" is the suffix for a nun. When a bushi died, his widow usually became an "ama" (nun) in an "amadera" (convent). Such a widow renamed herself, and put the suffix to her new name. Here are some famous nuns in Japanese history: Hoshun-in, Kenbai-in, Kensei-in, and Koudai-in. Tomoe changed her name to something else, and then added the "-in" to it when she became a nun. If you have a Japanese text-capable browser, here is the kanji for "Tomoe": 朋絵 The word "Tomoe" itself means "A Three Section Circle," or just "Circle".

It's pretty common knowledge that Tomoe was based very loosely upon Tomoe Gozen who lived and fought during the Gempei war of the 12th Century. What's not well-known is that her look was inspired by the Japanese actress Etsuko Shiomi who, unfortunately, is best known in the U.S. for a terrible performance in a dreadful movie called Sister Streetfighter, the third of the Shinichi Chiba Streetfighter series.

Shiomi appeared in a number of other productions, most notably the Kage no Gundan (Shadow Warriors) TV Series. Shadow Warriors lasted four seasons, each season with the same actors portraying a different generation of the Iga Ninja Clan fighting a new enemy (played by the same actor) who would be defeated at the end of the season -- so each season would be like a huge 26-part maxi-series. In Kage no Gundan II, shiomi portrayed Shiina Misato, head of the Shiina Clan, and it is from much of this performance which inspired Tomoe Ame. Shiomi also appeared in the 1987(?) remake of Satomi Hakkenden (Eight Warriors of the Satomi Clan). The original 1957 3-part movie is one of Stan Sakai's favorites and he's been trying to locate a copy of it for years now.