In an interview, Asagiri has said that Dazai is an BLANK character—"I think of Dazai as a donut. What's in the middle? No one knows. That's something that not even director Igarashi knows, or the voice actor for Dazai, Mamoru Miyano. In the middle of that unknown is the question, “Why does Dazai want to die?” That core aspect about him is why everyone gets pulled into the rest of the story. Although, now that I think about it, are you talking about Dazai, the author, or the representations of Dazai in Bungo Stray Dogs? Because I have thoughts on both.

Sure let's hear it. 

Dazai wrote a book called No Longer Human that expresses something that no other author has been able to get down on paper, which is the deep feeling of embarrassment that we can all relate to. Everyone is familiar with the feeling of embarrassment, but no one really expresses what that feeling is like or what kind of feelings it invokes. The image of Dazai is incredibly unique, and I wanted him to stand out in that way." 

Interview link:  If you are unable to see the link, go to bsd-bibliophileonlinelibrary, click interview and select writing relatable villain, there's a bug for some reason.

How does him being compared to a donut make him blank? The outer layers of a donut is tasty, everyone likes and more than often focuses on it rather than the inside because the inside is empty and there is no point of pursuing what is inside when its empty and cannot be eaten. This is the case for Dazai, as much as we question him, we cannot receive a proper answer, because there is no definite answer to our questions. Dazai's core element is blank, even if there is something in there, what can we do? Exactly, nothing.

Now onto your replies: "Dazai also definitely isn’t 100% up to interpretation. no fully fleshed out character is like that. you ‘Dazai is so complex!!!! he’s up to interpretation’ mfs r always so damn vague like?? WHAT" I never said he is a 100% into interpretation character, he has some aspects in his character in where people can understand some parts of his character, for example, his suicidal antics, his intelligence, and "all the things i don't ever want to be lost will always be lost."

"is up to interpretation? do you even know what interpretation is? do you even realise that you’re treating Dazai as a blank slate only there to change to your own views of it? do you realise that a ‘blank slate’ cannot be a complex character hence you are contradicting yourself? do you realise a ‘blank slate’ cannot be a fully fleshed character which Dazai clearly is?" Interpretation definition: the act of explaining, reframing, or otherwise showing your own understanding of something. "do you even realise that you’re treating Dazai as a blank slate only there to change to your own views of it? do you realise that a ‘blank slate’ cannot be a complex character hence you are contradicting yourself? do you realise a ‘blank slate’ cannot be a fully fleshed character which Dazai clearly is?" Because there's nothing for us to interpret him as, "why does he want to die? (his core element)" is it because he's bored of life? is it because he can't handle living any further? is it because he feels like a burden? Is it because he is unable to find the meaning to live? Is it because of an illness? Is it because he experienced abuse from his past? What happened to Dazai and what caused him to do this? We don't know, all we know is that he is a child who committed suicide at the age of 14 and was taken in by Mori as a "pawn". Why does he live in a shipping container? We don't know either, it could be simply a way for him to prevent assassinations towards him by mafia members, or it could be him isolating himself from everybody so when he dies, he won't have to worry about mortal means. We can go on and on about Dazai's character, and yet, we are unable to fully grasp upon his character, because there is nothing we can hold onto, all we are holding are pieces of sand slipping through our fingers easily.  

"that makes no fucking sense? “blank” CANNOT be complex. he is not fully up to interpretation you arguments make no sense & EVERYONE IN BSD IS MORALLY COMPLEX & GREY. YOU CANNOT TREAT DAZAI LIKE THE EXCEPTION. EXCEPTION. if we aren’t able to analyse him rationally and with logic then he ISNT A CHARACTER AND JUST A PLACEHOLDER, WHICH HE IS NOT. stop watering down his character to ‘up to interpretation’ too" Well no, Blank and Complexity can co-exist. Dazai IS complex, yet at the same time he is BLANK.. He hurts Akutagawa because he thinks its the best training for him, and yet, he knows that hurting akutagawa is not good (Contradictory Nature and Moral Ambiguity), In 18, he believed that the world is not a boring place and isn't worth living for, and that humans were just pawns, but when he met Oda and Chuuya, he changed! In the chapter where he got shot by fyodor and fyodor says "People are sinfully stupid. Even knowing they're being manipulated, they just can't stop killing each other. Someone must cleanse these sins.", Dazai replies, "Certainly, people are sinfully stupid, but what's so wrong with that?" I'd also like to bring up the fact that in PM he had an nihilistic view on humanity, and in current timeline, he follows the philosophy of existentialism. (Change in perspectives and character development), Dazai also had a unique outlook on death, he believes it is salvation of humanity. (Unique Perspectives)

Essentially, a BLANK character can BE complex under certain conditions, EVERYTHING IS POSSIBLE.

We cannot properly understand him with logic because his core element is hard to understand with the use of logic, and simply, the only answer you will receive from logic is that, "Suicide is not a good thing." Which is why we have to use psychological means on him, but if we do that, there is way too many reasons on why he may be doing this, hence, he CANNOT be understood properly.

"Dazai isn’t necessarily always misunderstood—rather no one TRIED to understand him due to his place in the mafia, and he believed he was outcasted due to his inhumane nature. although, yes, that is morals, human values/relationships, and what it is to be human in general, hence he searched for a reason to continue living, finding it in Chuuya because he showed him the ‘unpredictability’ of human nature, awakening a will to observe and understand within him. a lot of people who relate to Dazai simply relate to the isolation & lack of understanding/care given, rather than the whole humans thing" Correct, you are correct! but how does this answer the question of: Why does Dazai want to die? What happened to Dazai? Why does Dazai live in a shipping container? Exactly, NOTHING!! 

"huh? rephrase this sentence again ion understand it /nm. you claimed he is BLANK and UP TO INTERPRETATION, and I’m saying people IN THE SERIES struggle to understand him, not us as the viewer." then explain the interview. if that were the case, then people close to the manga would've been given an answer why Dazai acts that way. 

"i fucking hte NPD Dazai and generally cluster b Dazai except ASPD ngl. ion care what u hc but he definitely has the most ASPD symptoms…" aspd is a cluster b disorder.

In conclusion, you are stupid, and you just hate Dazai.