Mello (real name: Miheal Keehl) is one of the two successors of L and a major antagonist in Death Note. Although not truly evil, his methods of capturing Kira are downright insane and ruthless, down to the point where he even kills innocent people.
Mello was one of the people who grew up in the whammy house along with Near. He was meant to be a successor and wanted to avenge L when he heard of his death. After it was announced that L never chose who, he was told to work with Near to find out the truth about Kira. However, he refused as he did not want to work with anyone else and wanted to solve it on his own.
Mello has many similar features to L; he has a massive love for sweets as he's been shown to constantly eat chocolate bars. He also has an inferiority complex which makes him extremely arrogant and ambitious. Mello also has a very high intelligence, making him smarter than most people, the only exceptions being L and Near (and even Light Yagami). He tends to do whatever it takes to catch Kira and prove his superiority to everyone, even going as far as to kidnap innocents and use them for ransom.