Osamu Dazai (Bungo Stray Dogs)

Osamu Dazai (太宰 治, Dazai Osamu) is the deuteragonist of the anime/manga series Bungo Stray Dogs and one of the main protagonists of its prequel story arcs, Dark Era (with Sakunosuke Oda and Ango Sakaguchi) and Fifteen (with Chuuya Nakahara). He possesses an ability that nullifies people's abilities, titled "No Longer Human." At the age of 14, he attempted to commit suicide until he was found by the Port Mafia leader Ougai Mori to take him in as a useful insurance for the former's intelligence to kill the ill predecessor under the moonlight, and shortly after, he became Mori's executive and is regarded as the "Youngest Leader in the History of the Mafia". One year later, Dazai first started to work together with the new mafia executive Chuuya Nakahara, and their dynamic led to being titled as "Double Black" after killing an enemy organization as a whole. Three years later, he and Sakunosuke "Odasaku" Oda were tasked by Mori to find their colleague Ango Sakaguchi, who was recently disappeared. As Odasaku got shot by André Gide in the Mimic headquarters, his final words to Dazai was to go to the good side that saves people. Two weeks after his colleague's death, Dazai finally joined the Armed Detective Agency, led by Yukichi Fukuzawa.

Why He is the Most Resourceful Detective (and formerly Mafioso)

 * 1) He is the greatest villain-turned-hero protagonist who carries on Odasaku's important legacy to protect others and value human lives, as his colleague's final words encouraged Dazai to redeem himself by leaving the Port Mafia and join the Armed Detective Agency.
 * 2) He was the first person to value and care for Atsushi after the latter's years filled with pain and suffering in the orphanage, which is one of the greater aspects of Dazai's redemption as part of the Agency that protects humanity.
 * 3) * Not only did Dazai care for him, but also caring for another orphan Kyouka Izumi (also a former member of the Port Mafia), who inspired her to change her pessimistic perspective and gave her a chance to join the Agency as an ex-mafia assassin.
 * 4) Every character design of him in the anime, manga, and the promotional art is highly attractive, especially the ones from the majority of DEAD APPLE movie and the Cannibalism Aftermath party at the very end of season 3.
 * 5) * Mamoru Miyano's role as Dazai greatly delighted many fans of the aforementioned voice actor who is famously known for voicing iconic characters like Light Yagami from Death Note and Tamaki Suoh from Ouran High School Host Club.
 * 6) His rivalry dynamic with Chuuya Nakahara since the Fifteen arc is as memorable and well-written as that of Atsushi and Akutagawa's.
 * 7) Just like the non-ability user Ranpo Edogawa, he is highly intelligent and strategic to the point that he is greatly resourceful to the Agency (and to the extent, the Port Mafia, which was exploited by Mori to kill his predecessor in the past).
 * 8) Speaking of resourceful, he has an ability, titled "No Longer Human", that nullifies the activation of the users' abilities, which is the best for him to avoid any further violence or damage on others with powerful abilities like Ryunosuke Akutagawa's Rashoumon and Yumeno "Q" Kyusaku's Dogra Magra.
 * 9) On the comedic side of the series, his running gag of his suicidal attempts such as eating poisonous mushrooms and drowning himself in a river are hilariously entertaining.
 * 10) In addition, he has a great share of funny moments aside from his running gag, such as:
 * 11) * telling Chuuya to drink some more milk for being shorter than him.
 * 12) * revealing his previous occupation as a Port Mafia executive to Doppo Kunikida (who is the last person to know after everyone in the Agency).
 * 13) * flirting with the waitress in Café Uzumaki.
 * 14) As a man of wise words, his quotes are very unforgettable and thought-provoking:
 * 15) * "A good book is always good, no matter how many times you've already read it."
 * 16) * "Stop pitying yourself. Pity yourself, and your life becomes an endless nightmare."
 * 17) * "Justice is a weapon. It can be used to cause harm, but it cannot protect or save others."
 * 18) * "Evil expects evil from others."

Bad Qualities

 * 1) Even though his running gag is played for laughs, a series of his attempts to commit suicide may offend some audiences who value human lives.
 * 2) In the past as a Port Mafia executive, he was very abusive to Akutagawa, which is the reason why the latter became jealous of Atsushi for being greatly respected and approved by Dazai.

Trivia

 * Dazai's desire to commit double suicide with a woman is a reference to his real-life counterpart who attempted to do so until he finally did it with Tomie Yamazaki on June 13, 1948.