Just because a character keeps to themselves and doesn't talk much doesn't mean that they aren't powerful. In fact, some of the quietest characters are hiding the most dangerous secrets.
One great example of a strong but silent anime character is Bertolt Hoover, a seemingly innocuous person who is easy to ignore - until you find out that he's actually a violent menace who has a higher hit-count than almost anyone else around. There's also Hotaru Tomoe of Sailor Moon, whose silent demeanor hides her ability to obliterate planets with little effort.
Which quiet anime characters surprised you with their strength?
At first, Nagisa Shiota just seems like an ordinary, mildly depressed middle school kid who mostly keeps to himself. But when he begins learning the fine art of assassination from his tentacle monster teacher, he proves himself to be remarkably adept. His silent nature is especially useful because it gives him a leg up in sneaking up on his targets.
Unlike Yusuke and Kuwabara, Kurama is a quiet person who rarely talks about his life or his abilities. The combination of his polite, reticent nature and his pretty face often lead his opponents to underestimate him.
This is a fatal mistake. Not only is he an intensely strong person with high endurance, but his powers are also wildly dangerous. He can plant seeds in people's bodies that turn into parasitic plants, and his rose whip can flay his enemies to ribbons.
When Tokyo Ghoul begins, Ken Kaneki is an unassuming and introverted college student who spends most of his time reading and talking to his only friend. There's more to him than meets the eye, but it'll take some horrifically traumatic experiences to bring out his rawer, more violent nature.
After being transformed into a half-ghoul who is forced to eat human flesh to survive, being repeatedly attacked, and eventually tortured, his mousy personality transforms into a savage one.
Hinata Hyuuga might not be the most powerful person in the Naruto-verse, but she's a heck of a lot stronger than her gentle, soft-spoken personality implies. Hinata might have severe social anxiety, but she's also a powerful warrior.
She possesses the byakugan, an eye-based power that not only allows her to see for miles in every direction, but grants her the ability to make fine adjustments to other people's chakra - or to punch people with tiger-shaped balls of energy. It takes her some time to develop these abilities, but when she does use them, it's a spectacle. She's strongest when protecting her loved ones - especially the man she'll eventually marry, Naruto.