Mihael Keehl, universally known by his moniker Mello, is a key figure in the second half of the narrative. He was raised at Wammy's House, an orphanage in England founded by Watari for intellectually gifted children, where he was trained as one of two possible successors to the world's greatest detective, L. Unlike his rival Near, Mello is characterized by a fiery, impulsive, and emotionally driven personality. He operates with a fierce determination and a deep-seated inferiority complex, perpetually feeling second-best to Near, which fuels a burning resentment and a compulsive need to prove himself. His love of chocolate is a recurring quirk, often seen as a contrast to his otherwise hardened exterior. Mello's primary motivation is to capture Kira, the mass murderer who killed L, and to do so on his own terms, preferably before Near can succeed. This drive leads him to reject an offer to work with Near and instead leave the orphanage to join the American mafia, leveraging criminal resources to pursue Kira independently.

In the story, Mello serves as a wildcard and a secondary antagonist who inadvertently becomes instrumental in Kira's downfall. His aggressive tactics include kidnapping the director of the Japanese police to obtain a Death Note, then later kidnapping Sayu Yagami, the daughter of the police vice-director Soichiro Yagami, to successfully acquire the notebook. Through this, he meets the Shinigami Sidoh and learns about the existence of fake rules in the Death Note, information he later passes to Near. A raid led by Light Yagami on his hideout forces Mello to detonate explosives to escape, leaving him with a prominent facial scar and exposing his real name to Light. After this setback, Mello infiltrates the SPK to retrieve a photograph of himself, and then, with his friend Matt, spies on Misa Amane. His final act is to kidnap Kiyomi Takada, Kira's spokesperson, intending to use her to get to Light. However, Takada, possessing a hidden piece of Death Note paper, kills Mello by writing his real name. Despite his death, his kidnapping provokes Teru Mikami into using a hidden real Death Note to kill Takada, an action that Near notices, allowing him to swap the real notebook and ultimately expose Light's plan. Near later credits Mello as the most pivotal person in discovering Light's scheme.

Key relationships define his arc. His intense rivalry with Near is the core of his character: the two constantly attempt to outdo each other, and Mello's hatred of Near is cited as a flaw that occasionally clouds his judgment. His friendship with Matt is one of his few genuine bonds, and he demonstrates care for Matt's well-being. His relationship with Light Yagami is purely adversarial, with Mello acting as a persistent, unpredictable threat. His development shows a character who, despite his emotional volatility and moral ambiguity, possesses a sharp strategic mind. His abilities include exceptional intelligence, skill in manipulation and deception, and the capacity to command mafia networks. He is also resourceful in using the Death Note's rules to his advantage. His ultimate sacrifice, whether intentional or not, provides a critical piece of the puzzle that leads to Kira's exposure, cementing his role as a flawed but essential agent of justice in the story.