四季
It's a common thing in Japanese series (from what I've seen) to have main characters whose names are based off of the four seasons. After all, Japanese names are really easy to build from scratch. Here are the ones I remember:
作品
春
夏
秋
冬
「うらバン！浦和泉高等学校吹奏楽部」
コハル
ち
千
か
夏
あさみ
秋桜美
ふゆ
冬
み
美
「ケロロ軍曹」
?
なつ
夏
み
美
あき
秋
ふゆ
冬
き
樹
「四季おりおりっ！」
?
夏
希
秋
乃
?
「聖アドルフ学院物語～男の子だったら編～」
遥
夏
輝
秋
穂
冬
架
「初恋姉妹」
はる
榛
な
菜
ち
千
か
夏
あき
明
ほ
穂
とう
透
こ
子
「春夏秋冬（仮）」
はるか
なつ
夏
き
姫
あきほ
ふゆ
冬
か
華
「ファミリー・コンプレックス」
はる
春
み
海
なつ
夏
る
流
あき
秋
ら
良
ふゆ
冬
き
姫
「みなみけ」
はる
春
か
香
か
夏
な
奈
ち
千
あき
秋
?
「落花流水」
はる
春
か
河
み
水
なつ
夏
あき
秋
ほ
穂
ま
真
ふゆ
冬
As you can see, they're even tentatively naming one of the series after the seasons, too. So, anyone got any more?
[Edit (2007.04.30): Added Hatsu-Koi Shimai]
[Edit (2007.05.14): Added Family Complex (incidentally, by the same illustrator as Haru Natsu Aki Fuyu, under a different PN)]
[Edit (2007.08.28): Added to Family Complex]
[Edit (2007.10.12): Added Minami-ke]
[Edit (2008.12.02): Added from here and here, padding the earlier parts of the alphabet. It's kinda' pointless to add every one I encounter, but what the hell? I should also link to the authors (since they don't necessarily have series sites, most are linked to amazon, which doesn't have external links, presumably to prevent people finding competition), but I'm lazy. You should be able to get them by searching, or via the Yuri na Hi-Bi links in the first sentence fragment here.
[Edit (2009.11.14): The missing link from URABAN!-the first to be three characters, thus messing up my ruby. Also the characters from the male versions of HARUNATSUAKIFUYU, included in this otherwise female-only dōjinshi by the original authors; presumably they're read the same way as the normal ones. Interestingly, with the ones that were written in kana normally, the male versions are able to use the more common feminine characters and still be different.]