If you're looking for manga similar to The Detective Is Already Dead. Season 2, you might like these titles.
Kimihiko Kimizuka, a young man who four years ago became the assistant of a mysterious detective known as "Siesta" on a hijacked airplane. Together, Kimihiko and Siesta went on mind-boggling, globe-trotting adventures while combatting a secret organization, but all of that came to an end when they were separated by Siesta's death. In the present day, Kimihiko tries to return to an ordinary, boring life as a high school student, but things are never that simple, even though the detective is already dead.
The story centers on Minemori Kotoha, a woman who has been offered work at Suma Research Company's "Anti-Detective Section," and Sasaki Rena, its only investigator. This section focuses on rooting out corrupt detectives and removing them from their positions. While the two women spend their days investigating crooked cops, a new criminal is beginning to make his move...
There is a certain detective company on 7-chome Kagurazaka, Shinjuku-ku, Tokyo. Tsukioka Haruhi, Kannawa Yato, and Minakata Utsuhi. They are the rumored youkai boy detectives in the streets. You who are suffering from strange, weird, irrational, incomprehensible phenomena! If you come to this detective company, you will be immediately resolved and witnessed the monument!
A college student, Saitou Yakumo, is born with different eye colors. His red left eye gives him a special ability to see ghosts and spirits. He believes ghosts and spirits are bound to earth because of a certain 'cause,' and simply eliminating that 'cause' will let those spirits rest in peace. Being asked for help by Ozawa Haruka, they begin the investigation together.
When it comes to certain disappearance cases, police investigators seek advice from Shirou Tomura, a specialist in the history of play and leisure. After using his original and precise reasoning skills to unravel the truth, he's busy with his research and lectures, and yet requests from the police detectives seem to keep finding their way to him again and again... But the force that motivates him to continue is his own gushing inquisitiveness! So be careful when it comes to this eccentric gentleman's perilous fieldwork!
Set one year after the end of Persona 4, Naoto is asked to investigate the mysterious disappearances of two Yagakoro students by an old detective friend, Touko Aoi. There, she is partnered with Sousei Kurogami, a mechanical detective and a polar opposite to her in terms of personality and methodology. In their first encounter, she awakens to a new persona Amatsu Mikaboshi, which enables her to amplify Sousei's persona ability, Tsukuyomi. Together with Sousei, she discovers the Midnight Site, an anonymous site where judgement is passed on others by five judges named after figures in Japanese mythology: Uzume, Sarutahiko, Omoikane, Tajikarao, and Futotama.