lady_ganesh wanted to know about "Top five characters more dangerous than they look." Because proving this about a character always involves a reveal, there are spoilers (which I have covered) in each answer.
Cho Hakkai from Saiyuki (right). On the surface, a mild-mannered, absent-minded young man. But cross him, and you'll find out that he's totally bad-ass - even before he takes off his power limiters and becomes a youkai covered with prehensile vines and whose strength and reflex speed greatly exceed those of virtually every other youkai we've seen in the series.
Sanada Saemon-no-Suke Yukimura from Samurai Deeper Kyo (in the post icon), a cheerful short guy whom we first see as a harmless drunk with a woman on each arm. A few minutes later, he's puking his guts out. Who would suspect that he's one of the only swordsmen in Japan who can give the notorious Demon-Eyes Kyo a good fight ... and can chop lesser opponents in half vertically with a single stroke?
Yoshino Dōa from Blade of the Immortal (right), a very petite, very young Ainu woman who turns out to be a ruthless, blood-thirsty swordswoman with ultra-fast reflexes who takes out male opponents much larger and stronger than herself with fearsome efficiency.
Sister Yolanda from Black Lagoon, an elderly, sweet-faced one-eyed nun who is the Mother Superior of a church outside of Roanapur, Thailand, who packs a a gold-plated Desert Eagle pistol and is efficiently running an arms-smuggling and intelligence-gathering operation. (Sadly, I have no Sister Yolanda icon ... )
Ayasegawa Yumichika from Bleach (left), a fey, vain, eccentric little goofball who's too pretty for his own good and seemingly too refined to even perspire, yet in actuality is the Fifth Seat of the notoriously combat-ready and violent 11th Division of Soul Reapers, meaning that he can whip the ass of just about any even semi-normal opponent without ... breaking a sweat.