It's not uncommon to find anime characters of ambiguous gender but every now and again you need to check your eyes to see if what your looking at is really what it is. Rules this time will only cover cross dressers not anyone who has been turned into a girl so Ranma Saotome is instantly disqualified.
Update: Includes honorable mentions
Boy? Girl? or is Hideyoshi a Gender itself?
Honorable Mentions
Kuranosuke from Kuragehime
Gives away too many clues about his gender early on and it's obvious in the face.
Nuriko from Fushigi Yuugi
Not helped by the fact that pretty much every male is a pretty boy.
Naoto Shirogane from Persona 4
A rare female entry, but still pretty easy to figure out.
Haruhi Fujioka from Ouran Host Club
Again another rare female entry but extremely easy to figure out.
10. Ciel Phantomhive from Black Butler
Dressed in an extravagant ball gown to catch an evil count, Ciel manages to fool even his own fiancee. The dress is so famous that it's become a cosplayers finest challenge.
9. Kazuki from Getbackers
Not as convincing as the higher ups on this list but played well for comedy when he ends up in women's clothes; even more so he hates being mistaken for a homosexual.
8. Rosiel from Angel Sanctuary
I never got on with Angel Sanctuary as a series, but Rosiel certainly passes for this list to the point where a simple voice change would make him female.
7. Haku from Naruto
Mistaken straight away by Naruto and fans alike with some wishing he was female, even now it's difficult.
6. Loran Cehack from Turn A Gundam
Mistaken early on as a girl and dubbed "Laura" for a while. Made even more convincing during a convenient fancy party plot where Loran pulls off his disguise more convincingly.
5. Tieria Erde from Gundam 00
Speaking of convenient fancy party plots, in a possible homage to Loran, Tieria dons a fancy dress to meet up with the enemy; also goes the extra mile with fake breasts as well.
4. Hibiki Amawa from I My Me Strawberry Eggs
The whole plotline is him cross-dressing but manages the entire series without being discovered, he really cares about teaching that much.
3. Hideyoshi from Baka & Test
As you can see from the image, you decide: Male? Female? or is Hideyoshi simply a gender itself?
2. Ruby Moon from Card Captor Sakura
Not helped by the dub who made Ruby an actual girl, despite that it can still fool most people. CLAMP sure have a strange way of portraying men.
1. Aoi Futaba from You're Under Arrest
Originally cross-dressed to catch a criminal, Aoi took on the whole female persona to the point where he is essentially one of the girls. So much so that you probably need the writer to point it out to you.
