"Most ambiguous anime character"? How about "most ambiguous question"???
What does this question mean? Most difficult to figure out character in terms of behavior -- that is, you don't know what drives him or her? Character who seems to have the most ambiguous moral behavior -- that is, who seems to cross the line between good and bad so often that he's erased it? Character whose sexuality or sex is hard to figure out? Character whose SPECIES is hard to figure out?
So many possibilities.
In the end, I think I'll hit up one show for two different types, simply because there's so much ambiguity in the question, and the "motivation" bit can be a great deal a matter of debate.
And the lucky show is... Patalliro!
The first shonen-ai anime ever aired, Patalliro featured as its main characters the tall, dark, handsome (not to say bishonen) secret agent Bancoran of MI-5, his lover Mariach, and the title character, the boy king of Malynera, where most of the action takes place, an island kingdom whose main products (to quote Pat Munson-Siter) appear to be diamonds and prettyboys.
It's also one of the most Totally WRONG Anime ever made, in the break-your-brain Wrong sense, surpassed only by Bobobo-Bo-Bobo. My own (obviously non-canon) interpretation of the anime is seen here.
There appear to be virtually NO women on the island; we see exactly ONE (Patalliro's mother). How they maintain their population is a mystery.
Well, it's not at all uncommon for Western eyes to mistake a Bishonen for a girl. However, *everyone* mistakes Maraich for a girl. He doesn't have to change his appearance to disguise himself as a woman. The only thing that can possibly give him away is that he's ... well, flat. But the very few women we see in the show aren't very busty, either (the entire animation style is very attenuated anyway). He also *behaves* like a sterotypical woman. So he's awfully ambiguous.
And then he gets *PREGNANT*.
And his *son* (by Bancoran, of course) is a continuing character, so we're not talking about a delusion of pregnancy.
Yet canon statements are that Maraich is a man.
I personally won't believe it without full-frontal nude imagery. And no, I am not in any way ASKING for such images. No.
So Maraich is about as sexually ambiguous a character as I've seen. (I may well be very sheltered, of course)
And then there's... Patalliro himself.
Patalliro is supposed to be a boy king. Boy. As in, human. I AM NOT CONVINCED. He shapeshifts at will to any disguise he wants. In a world where people are routinely shot dead by bullets, he is often shot, stabbed, or beaten by his "bodyguards" Bancoran and Maraich (because, in a few words, Patalliro deserves it, being a perverted (even at age 8-10), arrogant, sneaky, arbitrary little twit who has no business being on a throne) and emerges none the worse for wear. He is a super-genius with an almost computer-like mind when he wants to use it (he rarely does), but mostly devotes his energies to tormenting the people around him. So I would put him in the "most ambiguous" category with respect to WHAT THE HELL *IS * HE???
Day 1 - Very first anime:Saint Seiya
Day 2 - Favourite TV series: : The Vision of Escaflowne
Day 3 - Your anime crush: Belldandy, from Ah! Megamisama/Ah/Oh My Goddess! and Shun from Saint Seiya
Day 4 - Favourite Film: Tonari no Totoro/ My Neighbor Totoro
Day 5 - Anime character you feel you are most like (or wish you were): Shikamaru from Naruto Shippuden
Day 6 - Most annoying anime character: Mihoshi, Tenchi Muyo!
Day 7 - Character you’d most like to slap: Sasuke, Naruto Shippuden
Day 8 - Most epic scene ever: End battle of YuuYuu Hakusho: The Poltergeist Report Movie
Day 9 - Saddest anime scene: Belldandy erasing herself from Keiichi's life in Ah! Megamisama/Ah/Oh My Goddess!
Day 10 - Favorite slice of life anime: Koko wa Greenwood
Day 11 - Favorite mech series: Giant Robo: The Day The Earth Stood Still
Day 12 - Best instalment of a franchise you don’t like: Nihao My Concubine
Day 13 - Worst instalment of a franchise you do like. Dragonball GT
Day 14 - Current (or most recent) anime wallpaper: Picture of Son Goku with Shen-Long done by my wife Kathleen
Day 15 - A reality check for a genre you like : Power-warrior anime and internal sanity/consistency!
Day 16 - An anime classic that simply doesn’t work for you: Neon Genesis Evanjellydonut... er, Televangelist...er, Evangelion
Day 17 - Best Background Music: Giant Robo: The Day The Earth Stood Still
Day 18 - Best Five-team: Yoroiden Samurai Troopers/Ronin Warriors (no link, too expensive)
Day 19 - Change you’d like to make to any one anime: Specific Change: Eliminate one nasty action from the history of Juraia-sama in Naruto/Naruto Shippuden. General Change: Eliminate the Saiyajin focus and expand capabilities and screen time for all major characters in Dragonball Z.
Day 20 - Most ambiguous anime character: Selected Maraich and Patalliro, from Patalliro!
Day 21 – Favourite OAV Series
Day 22 - Animation studio you'd most like to slap.
Day 23 - Anime you think had the best, or most intriguing art
Day 24 - Favorite anime hero or heroine
Day 25 - Best anime villain
Day 26 - Your favorite harem anime
Day 27 - Favorite anime opening theme song
Day 28 - Favorite anime companion/summoned creature
Day 29 - Best Anime Music Video
Day 30 - Favorite anime ending theme