*might contain spoilers*
Osamu Dazai is a member of armed detective agency and formerly he was a member of the underworld organisation, The Port Mafia. His ability is No Longer human, which allows him to nullify other abilities on contact.
Osamu Dazai is a mysterious person, he never reveals his true intentions until the time his right.Honestly, He is one of the most amusing characters I have come across. According to Kunikida,who is his colleague, Dazai is a lazy person who is always slacking off, and at first glance he looks like a character who's casual about everything and is a sucidal maniac, who's dream is to commit suicide with a beautiful women without experiencing any pain. As the story unfolds, we get the idea that Dazai also had cloudy days in his life, the dark era when he was one of the executives of The Port Mafia. Even then he was a suicidal maniac but he had a dark side to his character, though the suicidal scenes are comedic, somehow it showed ,as if he had a gap, a hole in his heart. The only person who he though was closest to him was his friend Sakunosuke Oda, who said to him with his dying breaths,"It can't be found...you should know yourself, whether you are on the side of the killing people or saving people, there will never be anything that can surpass your mind. No place in this world can fill your loneliness. You will linger in the darkness forever" . These lines changed him, not totally but he realised being on the side that saves people, not that it matters much to him, but gives his life a purpose.
Though after every ups and downs, Dazai is comic relief of the show, he is over dramatic which makes the scenes more lively and fun to watch. He mostly acts like a dumb person but deep inside he is clever and intelligent, so when things get serious, he's the one who is handling everything from the backstage since he is a type of person,who doesn't like to take credits. These variations makes his character more and more awe-inspiring. He has a sort of personality which is admired by the showgoers.
Also he cares deeply for his colleagues and juniors in Armed Detective Agency and for even those who are in Port Mafia. His relation with Kunikida on one hand, and Chuuya on another says it all. Since when Dazai was in Port Mafia,in the past, he wanted to commit suicide because he wanted to free himeself but presently he is lighthearted and the suicide scenes are comical.
If you are thinking he might teach you some ways to commit suicide without pain, you are way too wrong because he is a big failure in this.
Though Bungo Stray Dogs is a sort of anime in which each and every charcter has their own importance, but Dazai is one of my favorites and you are sure to like this character once you lay your eyes on him. Dazai is also one of the most important character as he greatly influences the lives of other characters like Akutagawa and Atsushi.