Aizawa has black hair and dark eyes. He is depicted as a tall man with a large afro. After the timeskip, he sports facial hair and a short crop of hair on his head.
Aizawa is an intelligent, respected member of the NPA and Task Force. He takes his work very seriously and for this reason does not approve of Misa Amane's antics. He becomes annoyed when she or someone else refers to her as Misa-Misa, and he often chides Matsuda for his immature behavior. Aizawa's respect for L is grudging, as Aizawa chooses to work within the law and L's methods of investigation are sometimes outside of the law. His contemptuous attitude intensifies after NPA funding for the Task Force is cut, when he conflicts with L over L's withholding of the vital fact that Watari would provide the Task Force with financial support if they chose to stay. He leaves the Task Force for a short while but returns before L's death. After Light Yagami becomes leader of the Task Force, Aizawa is the first member to suspect that Light is Kira. His lawful beliefs are exemplified when he stops Matsuda from killing Light in the final episode.
Aizawa is one of the NPA officers chosen to create the Japanese Task Force with the intent of catching Kira. When the NPA discontinues funding for the Task Force, L gives the members a choice of staying or leaving. If they stay with the Task Force, they will no longer receive a paycheck from the NPA, and because he needs to care for his family, Aizawa thinks of leaving. After Watari states that L had set aside money for the Task Force members for this kind of situation, Aizawa becomes furious. L reveals that he had not mentioned the money because he wanted to see what Aizawa would choose to do. As a result of this dispute with L, Aizawa leaves the Task Force.
He returns to the Task Force before L's death, and after the Task Force comes into contact with Near, he is the first member to suspect Light of being Kira. With Light's father in mind, he doesn't want to truly believe his suspicions. When Light begins holding audio-recorded meetings with Kiyomi Takada, Aizawa suspects that Light and Takada are passing notes and decides to mark the hotel note pads with a small hole. When he checks the hotel room after Light and Takada leave, he finds that the note pads present in the room do not have holes. This confirms his suspicions, as the hotel workers would have no reason to replace the note pads unless the others had been used up. Despite Aizawa's intelligence, Light considers him an idiot while they are following Takada after her kidnapping due to the fact that he and Ide took the front seat without thinking, allowing Light to use the piece of the notebook hidden in his watch to kill Takada. Aizawa shares his findings with Near, and his contributions to the SPK's investigation lead to Light's eventual defeat. He becomes the leader of the investigation team afterward.
Aizawa has a wife named Eriko and a daughter named Yumi. Aizawa and his wife also have a baby son whose name is not revealed. Halfway through the series, it is implied that Aizawa and his wife were having marital problems as a result of Aizawa's working on-call around the clock and rarely being home. Aizawa's daughter and wife both seem to miss him, and Yumi is happy whenever he comes home. His wife Eriko, however, tends to take it in a more hostile manner. He is shown in a park, where by chance he sees his family; he begins to cry while holding his daughter. It is unknown whether he and his family were able to mend, as they are not mentioned by Aizawa after the timeskip.
Aizawa's role in the film is more or less the same as it is in the manga and anime, although some large changes were made. Aizawa doesn't suspect Light of being Kira as he does in the manga and anime, the plot point regarding Aizawa and Near is absent (Near is not in the films), and the issues with his family are primarily absent.
Obata said that he did not plan to draw Aizawa for an extended period of time, so he "began to worry" when Aizawa continued appearing "more and more." Obata felt that originally his face was not "good for a main character" so he said he decided to "gradually make him more presentable. [laughs]" and increase the volume of his hair. Obata believes that Aizawa was "easy to draw" and that it is easy for him to draw people who "act consistently." Obata said that he drew Aizawa while believing that "it would be nice if there were lots of people like him in Japan." Obata redesigned Aizawa to "look older" and "really make it feel like four years had passed since L's death."
Towards the end of Death Note, Tsugumi Ohba decided to make Aizawa the chief of the NPA because he did not want to create a new character in the final chapter. Ohba felt that Aizawa is the best person to be the NPA head because Aizawa held connections to the Japanese police and knows Near. Ohba also said that whilst the other former Kira Investigation Team members had the same qualifications, he did not want to use Mogi due to his lack of personality development, nor Ide because he was so recently re-introduced and readers knew so little about him. Then he laughingly added, "Matsuda was out of the question! Maybe after Aizawa retires? We will see if he matures..." Obata then asked Ohba: "Do we make a new manga for just Matsuda?" And Ohba replied, "Of course not. Baka [Idiot] Takeshi-san." [Note: Ohba was joking with Obata, making a reference to the phrase often used by L, Light, and Aizawa: "Baka Matsuda!" [In English dub, "Matsuda, you idiot!"]
- In the anime, certain characters take on stylized hair colors while the viewer hears their thoughts; Aizawa's color is green.
- “Would you cut it out with all this dating, kissing and Misa-Misa talk already?! This is the Kira Investigation! Stop messing around!” (Episode 17)
- “Mogi, Ide. Help me arrest Kira, Light Yagami, the mass murderer.” (Chapter 107)
- (when Matsuda attempts to approach a dying Light) “Matsuda! Stop it. What is it this time, sympathy?” (Chapter 107)
- (various times throughout the anime) “Matsuda, you idiot!”