Kurai Motoki

Kurai Motoki is a shinobi of, you guessed it, the Motoki clan, and is probably the only one, other than Hinata who actually thought Naruto was a cool guy.

Background
Kurai is a member of the Motoki clan, a very powerful clan that has a pretty dark reputation with a few other clans, yeah they were pretty much hated by everyone. Eventually they were either exterminated by the nine tailed fox, I think they made it mad to... *Sigh* or they went into hiding after the attack, the latter is more likely, but one day, a superstitious Uchiha, before they were exterminated gathered a few friends and found Kurai's family, they caught them by suprise and Kurai found himself face to face with about a dozen angry Uchiha's, but then something inside of him snapped. He just witnessed the death of almost his entire family and his rage built up inside him, you know how I said that the Motoki clan was very powerful, it has something to do with their Kekkei Genkai. the Akumagan which literally translates into demon, with "gan" at the end, more explanation on the Akumagan later. He used this power to obliterate the murderers, and passed out outside the village gate, and ended up living with Naruto, and acting like his older brother. He later joined the academy and met a girl called Minako, when she was being bullied by a group of kids, who almost got obliterated by about 5 kicks in the balls each. He also met another guy called Katsu, who ended up being his rival for a long time, and they ended up being on the same squad as ninja.

Personality
Growing up with Naruto, Kurai has a happy go lucky personality, but he also has a serious side, y'know with him dealing with the slaughter of his family, but he doesn't really remember it that much, and when he does he kind of goes on a rampage and destroys almost everything around him... yeah. But he does like to joke around alot and has helped Naruto with a few of his pranks in the past. He also has a very strong desire to protect his friends and will do anything to save them, including killing his opponent.

Kekkei Genkai
The Akumagan is a kind of transformation/technique that multiplies your speed, strength and chakra depending on how angry you are or how much control you have over your chakra, but it chews your chakra very quickly so it is very dangerous to use un-responsibly, which is what has made it kill many users, as people usually humiliate them when they are low on chakra.

Sensory Perception
Kurai can sense chakra over long distances, well beyond the range of other Shinobi. His ability is so acute that he can read the emotions of faraway ninja's as if he were right next to them. Personalities and power can easily be determined from the male with only a small bit of effort, as well as a chakra gauge of others limits.