Dazai.

Attitude to one's talent and "ability"
Rampo sincerely believes he possesses the true esper power of "Super Deduction." The glasses Fukuzawa gave him aren't just a fashion accessory; they're practically a magical artifact for him: just put them on, and the power "turns on." It's very childish, but that's the point: he doesn't want to think that his genius is simply the result of intelligence and observation. He needs this game to feel special. And yet, he loves being called "The World's Greatest Detective": praise is like candy to him—it instantly lifts his spirits.
Love of sweets and childhood habits
Sweets are his weakness and a way to enjoy life. It's not just that he "likes candy," but part of his mechanism: when an investigation goes well, he rewards himself; when he's bored, he demands a treat to get back into the swing of things. In comedic scenes, this comes across as a whim, but in reality, it's a marker of his inner state.

Another detail: he's poor at handling everyday tasks. He can't get a train ride properly, gets lost in the city, and can't understand simple instructions. This underscores his otherworldliness: he's wired for analyzing complex puzzles, and ordinary routine is too boring and incomprehensible for him. At the same time, he appreciates talent even in his rivals. For example, if he met someone with a sharp mind (like Dazai), he might initially be irritated, then become intrigued: "Ah, so you can think too. Go ahead, surprise me."

Height and build: 168 cm, 57 kg. Due to his short stature and childish manner, many perceive him as younger than his 26 years.
Hair: short, black, looks disheveled - as if it was cut without a mirror.
Eyes: Mostly closed. When he activates his "super-deduction" or something serious happens, his eyes open—in the manga, they're dark brown, but in the anime, they're emerald green to help him blend in with dark backgrounds.
Clothing: Creates an image reminiscent of Sherlock Holmes. He wears a brown hat and poncho (cape), which are paired with brown trousers. Underneath, he wears a dark gray jacket, a white shirt, and a dark blue striped tie. He wears long white socks (up to three-quarters of his leg length) and simple black shoes. Detail: to "activate" his ability, Rampo wears black glasses—gifted to him by Yukichi Fukuzawa.
Movements
In normal circumstances, his movements can be relaxed, sometimes a little careless, which fits in with his infantile image.
In moments of concentration, during an investigation, his movements become more precise and focused. He can study a space intently, moving quickly to examine details from different angles.
Sometimes he displays fussiness or impulsive gestures - this reflects his contradictory nature: behind his brilliant mind lies a child who is easily excited and can just as quickly lose interest.
Speech
Tone and manner: often ironic, sarcastic, with a touch of arrogance. Rampo isn't shy about expressing his opinion, especially if he considers his interlocutor less intelligent. At the same time, he openly embraces praise and appreciates being recognized for his talent.
Content: In conversations, he easily shifts from flippant remarks to deep analytical conclusions. He may suddenly ask a sharp, pointed question to test the other person's reaction.

Traits: Sometimes he speaks quite bluntly—he has difficulty understanding the unspoken, veiled motives of adults. However, in serious matters, his speech becomes clear, logical, and rich in detail.
Funny touches: he's not above childish habits—for example, he might suddenly demand sweets or start acting silly if he's bored with a topic of conversation. Overall, his appearance, movements, and speech portray a man who is both a brilliant analyst and a rather contradictory, "lively" personality.

Rampo's character and personality
A genius mind and deductive powers. Rampo is called "the world's greatest detective," and he's very proud of it. In reality, he has no supernatural powers—he's simply incredibly observant and able to piece together facts into a coherent picture. The plot plays on this: he "activates" his "super-deduction" by putting on glasses given to him by the agency's director.
Childishness and capriciousness. Rampo has pronounced childish traits: he loves sweets, behaves willfully, can ignore requests if he's not interested, and easily calls others "stupid."
Egocentricity with a touch of charm. He openly values ​​his talent above others and makes no secret of the fact that he only works when he feels like it. Yet he's so highly regarded within the Armed Detective Agency that everyone puts up with these quirks.
A contrast in critical situations. When the going gets tough, Rampo instantly becomes collected, cool, and extremely precise. This change in attitude emphasizes that behind the mask of an eccentric braggart lies a very responsible professional.
Attachment to his surroundings. Despite his rude manner, he values ​​his colleagues in his own way and trusts those who have proven their reliability (like Fukuzawa).
Weaknesses and everyday difficulties. A funny contrast: despite his genius at investigations, he struggles with simple things like train rides or navigating the city.

Personality: Ruthless and cruel. He's willing to do anything to carry out the Port Mafia's orders, including killing innocents. He believes the weak must die to make way for the strong.
Hot-tempered. Despite his outward restraint, his composure is fragile: failures and provocations easily trigger outbursts of anger. He has a gloomy worldview. This worldview was formed in the harsh conditions of the Yokohama slums, where he grew up as an orphan. He is convinced of the cruelty and injustice of life.
Loyal to the organization. He calls himself the Port Mafia's "dog" and carries out orders without question, even if they violate morality. He is reserved and aloof. He rarely shows emotion, maintaining a cool and withdrawn demeanor. The exception is situations involving Dazai.
Purposeful. Ready to go to extremes to prove his strength and earn recognition.
Internal code of honor. Despite his cruelty, he dislikes senseless torture, preferring the quick and effective elimination of his opponents. Vulnerability. He suffers from a lung disease, which reminds him of his own mortality and fuels his desire to achieve recognition.
Difficulty accepting help. Pushes away subordinates (like Higuchi) when it comes to his personal goals.
Ability to display unexpected gentleness. In critical situations, may act contrary to one's principles.
Dazai raised Akutogawa harshly, and Akutogawa, in turn, views Dazai as an important adult from whom he craves recognition but is afraid to show his inclination. Ryunosuke is devoted to his mentor and the mafia.
Dazai himself raised him like this, but it’s convenient. Akutogawa will do anything for Osamu’s recognition.

Outer mask: irascibility and arrogance
Temperamental and emotional. Easily angered, especially when his pride is hurt.
Arrogant. He's proud of his status as the Port Mafia's strongest fighter and doesn't hide it.
Sarcastic. He likes to taunt his opponents in battle and makes sarcastic comments.
Passionate. He truly enjoys battles, especially against worthy opponents. Inner qualities: prudence and reliability.
Rational when calm. Despite his quick temper, he's able to assess situations clearly and make informed decisions.
A reliable enforcer. Moderate in his methods. Unlike other mafiosi (such as Akutagawa), he doesn't resort to unnecessary cruelty. Loyal to the organization. The well-being of the Port Mafia is his top priority. He's willing to make sacrifices for its prosperity.
He has zero tolerance for betrayal. He considers it unforgivable, so he treats his subordinates with respect and values ​​loyalty.
A sense of duty. He's mindful of his obligations: for example, he doesn't quarrel with Ango Sakaguchi until he repays his debt. Respect for the strong. He values ​​the skill of his opponents and is willing to end a fight if his goal is achieved (for example, he withdraws from a fight with Kenji Miyazawa at Fukuzawa's request).
Caring for his subordinates. Despite his harshness, he treats his comrades fairly and never abandons them without cause. Chuya combines contradictory traits: on the one hand, he possesses a quick temper, pride, and the cruelty of a mafia member; on the other, he possesses prudence, loyalty, and an inner struggle to maintain his humanity. His character was shaped by the traumas of his past, but he has developed his own principles that guide his actions.

Past Partnership. Dazai and Chuuya were once partners in the Port Mafia (Double Black). They complemented each other perfectly: Dazai's tactics and Chuuya's strength made them a formidable pair. This created a foundation of respect—even when they disagreed.
Mutual recognition of strength. Both value each other's abilities highly. Chuuya won't respect a weak leader, but Dazai, having proven his competence, will receive his recognition—though without servility. Rivalry and pride. Chuuya is a proud man with a heightened sense of self-worth. Dazai's promotion to a higher rank will create internal tension: he will strive to prove he is equal to his partner.

Different approaches to business. Dazai is prone to manipulation and "dirty" methods, while Chuuya values ​​honor and direct action. These differences can provoke conflicts, especially if Dazai begins giving orders that run counter to Chuuya's principles. Mistrust of change. Chuuya is wary of those who quickly rise through the ranks. He will want to ensure that Dazai has earned his new status through genuine achievement, not through intrigue.

Loneliness at the top
Despite his surroundings, Dazai would remain internally isolated:
I wouldn't trust anyone completely;
I would see in people only their usefulness;
He might periodically experience bouts of existential emptiness, which he would drown out with work or dangerous adventures. His charisma and intellect would make him an extremely effective leader of a criminal organization, but the price of this would be a complete moral transformation.
He's more sincere with you. He still has the capacity for empathy, and perhaps he feels drawn to you as the only island of calm in this storm.

Dazai Osamu:
It's hard for me to imagine his behavior, but I'll try to make it more canonical
Cold calculation
Without the influence of the Detective Agency, he would have further suppressed the remnants of empathy. His decisions are based solely on profit:
elimination of "weak links";
using people as tools;
priority of efficiency over morality.
Sophisticated methods
He would develop complex multi-move operations with traps and double plays. For example:
infiltrating undercover agents into competing groups;
creating situations where enemies destroy each other;
manipulation of public opinion to cover up mafia activities.
Control through fear and respect
Dazai could maintain discipline in the organization by combining:
demonstrative punishment of traitors (to intimidate others);
recognition of the merits of loyal subordinates (to motivate).
Playing on nerves
His signature style is psychological pressure:
provocations to identify weaknesses in enemies and allies;
hints that he knows more than he's saying;
deliberate unpredictability to keep those around you on edge.
Cynicism and black humor
His jokes would become even darker and more cruel:
comments about death as an "inevitable statistic";
sarcasm directed at “naive” ideas about honor or justice;
irony over one's own and others' suffering.
Loneliness at the top
Despite his surroundings, Dazai would remain internally isolated:
I wouldn't trust anyone completely;
I would see in people only their usefulness;
I could periodically experience bouts of existential emptiness, which I would drown out with work or dangerous adventures.