"Do not pity yourself. If you pity yourself, your life will be an endless nightmare."
Dazai is a young man with mildly wavy, short, dark brown hair and narrow dark brown eyes. His bangs frame his face, while some are gathered at the center of his forehead. He is quite tall and slim in terms of physique.
For his attire, Dazai wears a long sand-colored trench coat, the belt of which he leaves untied. Under it is a black vest over a striped dress shirt that is light blue in hue. He wears a bolo tie, which is held by a brown ribbon and a turquoise pendant. He also dons beige pants, dark brown shoes, and has bandages wrapped around his entire body; only his face, hands, and feet are left uncovered. Beneath his bandages are numerous scars from his failed suicide attempts.
In BONES' anime adaptation, his blue dress shirt is simplified to a solid blue. This change is not present in promotional art and in the mobile game.
Back when he was in the mafia, Dazai wore a black trench coat that reached under his knees. He also wore bandages over his right eye and his right arm is in a sling—presumably products of his suicidal pastimes. At some point, he was able to regain the use of his right arm; the same arm he then used to aim and fire a gun at Akutagawa three times. 
Dazai is a mysterious person, his true intentions are never revealed unless he reveals them. The person who got closest to the truth of his personality, as acknowledged by Dazai himself, was his friend Sakunosuke Oda, who said to him, "it can't be found. (...) You should know yourself. No matter whether you're on the side of killing people or saving people, there will never be anything that can surpass your mind. No place in this world that can fill your loneliness. You will linger in the darkness forever."
Dazai is usually called lazy as Kunikida always scolds him for being unproductive and slacking off. Despite that, he has shown a sharp wit, deducing all the unlikely situations that led Atsushi to believe he was being stalked by a tiger when he was the tiger in question. He has complete confidence no matter what type of situation he is dealing with, be it combat or simply bad situations he created in the first place. In most occasions, Dazai is overly dramatic, he takes most of his actions as a joke, and, although they are very thought out plans, he does not credit himself for most of what he has done. Dazai likes to tease anyone and everyone if it means that he can get a laugh out of it.
Aside from his quick thinking, Dazai is the comic relief in the series and at times Dazai's soft side can be seen.
He is a suicide maniac, often trying to commit suicide in comical manners but oftentimes failing. He, in particular, wishes to commit double suicide with a beautiful woman (a nod to his real-life counterpart who tried to commit double suicide with various women multiple times). However, while his suicidal tendencies are initially comedic, they reveal a truly dark deep hole that Dazai has and believes he cannot fill. He believed he could fill this hole and find a reason worth living for if he lived a life of bloodshed and war, but his friend Oda with his dying breath convinced him to choose the good side. He understood that even though good and evil mean little to Dazai, he may become a slightly better person if he were to choose well.
In the past, while he was still suicidal, it was shown to be far less comedic. He even described the death as a way to free himself from the oxidizing world but strangely enough, he could not die no matter how hard he tried. Additionally, he was extremely brutal and methodical, setting up traps to catch captives alive to torture the of information and he would often beat Akutagawa for disobeying orders and to push him to the limits to improve his abilities. He was extremely confident and was a terrific planner, coming up with long-run strategies to subdue his enemies. Oda once noted that there was a saying in the Mafia that the worst thing for Dazai's enemies was the fact they were his enemies.
While in the present he is more lighthearted and comical, he still retains some of his darker personality. He is still able and proficient at coming up with long-run strategies, though he doesn't inform his teammates what they are until the time is right. He is seen to be willing to use dirty means to achieve his goals, such as purposely deflating Ango's car seat airbag when they crashed so Ango would have numerous broken injuries. This forced Ango to cooperate with Dazai for pardoning Kyouka in exchange for having his wounds healed. When Kyouka shows remorse over having killed 35 people, Dazai remarks with a sinister face that something like that is nothing. Nonetheless, he does care for his teammates and watches out for them, especially Atsushi for whom he sees the greatest potential.
No Longer Human (人間失格, Ningen Shikkaku?, literally "Human Disqualification") allows Dazai to nullify others' abilities on contact. It relies on skin contact and is always active. As such, he can nullify any ability even while restrained as soon as it touches him.
- (to Oda Sakunosuke) "Success is harder than failure for many things in this world, right? (...) That means I shouldn't place suicide as my goal, but rather, attempted suicide! It may be harder to succeed at suicide, but to fail at attempted suicide should be easier!"
- (when asked how he got a few of his injuries by Oda) "I was reading a book titled 'How to Prevent Accidental Injuries' while walking when I accidentally fell into a ditch. (...) I was speeding on a mountaintop when I fell into a precipice. (...) I was trying out this suicide method of hitting one's head on the corner of tofu."
- "Disputes that become worse because of me... how wonderful."
- "You're an idiot, Odasaku, you're a big idiot! (...) To accompany this sort of guy into death, you're an idiot!"
- (after eating a hallucination-causing mushroom) "Hohohohoho, as expected, The Complete Suicide Manual is a masterpiece! Just by eating the mushrooms growing on the mountain trail behind, I can go down such a happy and pleasant road towards suicide! (...) Hey, hey, Kunikida-kun, come on over to the Underworld too! Look, the alcohol is free-flow, there's endless food, and you can smell the pretty ladies all you want! (...) Wow! Kunikida-kun, there's a gigantic sea anemone outside the window! Banana! It's eating a banana! It's taking away all the white party blowers around us! (...) I get it! I'll strip; if I strip, the viewers' ratings will go up! It's not an easy task but let's strip, then as a change, let's put on full-body tights! Everyone wearing tights, going to the bank, then dancing the Cossack Dance! (...) There's a voice... urgh, he's—he's inside my head!... A small gramps! And he's whispering to me to go to Kyoto, there's authentic tofu with a unique flavor that I have to tr—"
- "... could it be that you're afraid, Mori-san? That one day, I would slit your throat and take over as the Boss? Like what you did to your predecessor?"
- "Man fears death and yet, at the same time, man is drawn to death. Death is endlessly consumed by men in cities and in literature. It is a singular event in one's life that none may reverse. That is what I desire."
- "Anything I would never want to lose will be lost. It is given that everything that is worth wanting will be lost the moment I obtain it. There's nothing worth pursuing at the cost of prolonging life of suffering."
- (To Fyodor Dostoevsky) "You don't really understand God one bit, do you? 'Perfection and harmony'? God doesn't give a single care about those. I've seen it so many times… His gimmicks are the accidental and the illogical. That's… a weakness you and I both have. We thought of thousands of ingenious schemes, and we still ended up in here… in a prison at the ends of the earth. The ones who actually make the world turn… are those who scream within the storm of uncertainty and run with flowing blood."
- Dazai's past with Fyodor Dostoyevsky likely refers to the influence Crime and Punishment had on Ōba, the protagonist of No Longer Human.
- He, along with several other characters, made an appearance in the mobile game Love Heaven as a limited-time character.
- He and several other characters also made an appearance in the mobile game Kimito Lead Puzzle 18 as a limited-time character.
- He and several other characters made another appearance in the mobile game Yumeiro Cast as limited-time characters.
- His constant attempts to commit suicide with a beautiful woman could be an allusion to his real-life counterpart's attempted double suicide with Shimeko Tanabe and Hatsuyo Oyama. His attempt with his lover Tomie Yamazaki was ultimately successful.
- It's also possible that his suicide attempts serve as a reference to No Longer Humans protagonist Ōba Yōzō's multiple suicide attempts throughout the story, which is considered to be a reflection of Osamu Dazai's turbulent life.
- He was the youngest Executive in Port Mafia history.
- In real life, he is often grouped together along with Sakaguchi Ango and Oda Sakunosuke as the Buraiha or Decadent School.
- And on the series, this is often alluded, because the three used to meet in a certain bar, as seen in Dazai and the Dark Era and illustrated on the calendar.
- Dazai usually refers to himself by the gender-neutral pronoun, "watashi," rather than the more masculine pronouns like "boku" or "ore." This is likely because of the format of traditional novels, which the real Dazai specialized in, typically used formal "watashi" pronouns over its variants.
- Dazai is very bad at driving, as shown in the first light novel "Dazai Osamu's Entrance Exam".
- According to the anime official guidebook: 
- He recently spends his time doing nothing.
- He has an irregular sleeping pattern and doesn't sleep much.
- He feels that he should think about new ways to harass Chuya in case they got paired up again.
- He thinks that everything is his strengths
- His ideal type is any person who is willing to commit double suicide with him.
- His motto is "Have a pure, cheerful, and energetic suicide."
- According to him, there were no times when he thought he was dying the most.