Post a character whose gender is not clearly reveal until later on, at the very end, or never reveal at all and overall ambiguous

Elizabeth from Gintama :3

Nuriko from fushigi yugi!!! :D

Yoite (Nabari no Ou) was born intersexed and he himself isn't sure if he's a girl or a boy.

In the show it's never mentioned what gender Kaoru is.

It's up to our imaginations~ XD

Charles or Charlotte

Haku (Naruto)

Naruto thought Haku was a girl....also he also commented her telling beautiful in his mind..but after their conversation when they part..........Haku tell Naruto that he is a boy.....and Naruto was shocked.......he he eheh

Frosch

DogKeeper (No6) - She's treated and acts like a guy. It's not until toward the end that Shion realizes she's a girl. :)

In Shugo Chara! we are first introduced to Nadeshiko and then later "her twin" Nagihiko. While the audience knows later they are really the same person, Amu doesn't. And we find out that she is really a he. Even he is a little confused because he has two different "gender" character eggs.