An orphan that grew up with Near in Wammy's House, a shelter for gifted children overseen by Watari. He's a candidate to be L's successor. He also really likes chocolate.
Mello's real name, Mihael Keehl; was raised at the Wammy House as a candidate to become the next successor of L. He and his rival Near were asked to work together during the Kira case. Due to Mello's stubborn personality, he ran away and worked on the Kira case with a Mafia gang until his death. Mello is nowhere near reserved, and he speaks out whenever he can. Unlike Near and L who are both calm and collected, Mello is very rash and emotional. He has the tendency to act before thinking things through. This is what makes Mello so dangerous - is that he is extremely unpredictable. This has caused him to become a huge threat against Light, when Mello gets his hand on one of the Death Notes. Mello has a wide variety of connections, and if he wanted to he could get all of his work done among the shadows just by using people to complete tasks for him.
Mello has a huge inferiority complex against Near, and has a very strong desire to prove himself. The level of his intelligence is incomprehensible. One of his best abilities is to act fast, and to think beyond the fetters of the law with his concepts and actions. All Mello's life, he wished to become number one and surpass Near. Everything to Mello was a challenge for him; despite the fact that knows he cannot make it to the top. During the Kira Case, Mello's irrational personality eventually caused him to become a criminal [the kidnapping of Souichiro Yagami's daughter, Sayu and the kidnapping of Takada].
Mello kidnapped Yagami Sayu in exchange for the Death Note. But, in order to get the Death Note back, Yagami Light sets up a raid to invade Mello's hide out. This leaves Mello badly scarred, because Mello's only choice to escape from the raid was to blow up the building. So, in the end Mello made it out alive, but recieved a large scar and revealed his real name to Souichiro Yagami (who had the Shinigami eyes at the time).
Mello's death occured during the time he had kidnapped Takada in order to lura Kira closer to him. He had his friend Matt lead Takada's body guards away from Mello, by using smokescreen and a getaway car. Whereas Mello handcuffed Takada to himself, and sped away on his motorcycle. Mello drove themselves over to a large truck. The two climbed into the back, and Mello had ordered Takada to remove all of her clothing to avoid any more suspicions. But, Takada had a piece of the Death Note tucked away in her underment. And she wrote down Mello's name while he was driving the truck. Thus Mello had died due to a heart attack.
From "Death Note : How to Read":
Real Name: Mihael Keehl
Also Known As: Mello, M
Birthdate: December 13th, 1989
Deathdate: January 1st, 2010
Height: 171 cm (5'6")
Weight: 52 kg (114 Pounds)
Blood Type: A
Dislikes: People who are better than him
Has a Death Note: No
Ability to Conceptualize: 7/10
Willingness to Act: 10/10
Social Life: 9/10
Inferiority Complex: 8/10