FBI Director Steve Mason is a supporting character in Death Note who serves as the head of the Federal Bureau of Investigation. His background is defined by his high-ranking position within the FBI, a role that places him at the center of international law enforcement. He becomes involved in the Kira investigation when the world-renowned detective L suspects that Kira may be connected to the Japanese police force.

Mason's personality is marked by a combination of initial decisiveness and subsequent self-preservation. He demonstrates a willingness to cooperate with L's unconventional methods, agreeing to a risky and clandestine operation. However, when the stakes become personally threatening, he shows a strong instinct to protect himself and his organization from harm. He is portrayed as a pragmatic leader who, when faced with catastrophic failure and an invisible, untouchable enemy, prioritizes institutional survival over the continuation of a dangerous investigation.

His primary motivation shifts as the story progresses. Initially, he is driven by a professional duty to assist in capturing a mass murderer. After the devastating loss of his agents, his motivation changes to fear and accountability. He becomes desperate to distance the FBI from the case to avoid further casualties and to shield himself from political and professional repercussions. Later, in the second part of the story, his motivation appears to be a renewed commitment to stopping Kira, this time by supporting a new, more secretive task force.

In the story's first arc, Mason's role is crucial. Acting on a request from L, he dispatches twelve FBI agents, including Raye Penber, to Japan to conduct covert surveillance on the Japanese police. After Kira identifies and kills every single one of these agents, Mason, terrified of being held responsible and becoming Kira's next target, terminates the FBI's cooperation with L. He further complicates the situation by publicly revealing that the FBI had been investigating the Japanese police, creating a major setback for L's investigation.

In the second arc, following L's death, Mason resurfaces as a member of the SPK, the Special Provision for Kira, a covert American-led organization formed to apprehend the new Kira. He works in support of Near, L's successor, providing resources and assistance to establish the new task force. His key relationships are therefore primarily professional. He has a working relationship with L, which he ends abruptly. Later, he forms a cooperative alliance with Near, which ultimately proves fatal. His entire organization is an adversary to Light Yagami, Kira, who sees the FBI as a major threat to be eliminated.

Mason undergoes a distinct development, though his screen time is limited. He transitions from a confident official who can lend significant resources to a global investigation, to a frightened bureaucrat trying to salvage his career and life from a supernatural threat. His final role, as a member of the SPK, shows a return to the fight against Kira, but in a supporting rather than a leading capacity. This arc ends in tragedy, as he is killed by Mello, a rival to Near, becoming a casualty in the complex war between the different factions trying to control the Death Note.

Mason does not possess any supernatural or exceptional detective abilities. His notable abilities are entirely institutional. As the FBI Director, he commands a vast network of resources, agents, and information, making him a powerful ally. His authority allows him to authorize large-scale, cross-border operations, which is exactly the kind of asset L needs. However, his power is also shown to be limited; he cannot protect his agents from a force as indiscriminate as Kira, nor can he withstand the political and personal terror that Kira inspires.