Mori is a forty-year-old man, with a fairly tall and slender physique. His eyes are violet, and befitting of a mafia boss, almost always completely dark and devoid of light. He has straight, chin-length black hair slicked back, leaving bangs on each side of his face.
Normally, he wears a sharp suit, consisting of a white button-up, a black pinstriped suit jacket, black shoes, and black trousers. The shirt has a purple and white striped tie. Over this, he wears a black trench coat and long, almost knee-length maroon scarf he leaves hanging over his shoulders untied. He also wears white gloves.
In public, Mori wears much less lavish attire, more typical of a "doctor". He wears a purple button-up shirt, black tie, black trousers, black shoes, and a long white coat. Furthermore, he leaves his hair loose, and two strands fall over his forehead.
While initially showing himself as an ordinary, clumsy and worried middle-aged man, he later shows more about his true character, keeping calm even when the other people trapped in Anne's room ran away, staying together with Atsushi and Jun'ichirō Tanizaki, and still maintaining his composure when the latter was captured.
Mori later reveals that not only is he good with strategies and planning, but also a cruel and merciless person capable of infusing fear on his enemies. However, his cruelty is not for cruelty's sake. Mori always weights the gains and losses of his decisions, choosing the optimal solutions for the Port Mafia as a whole. Consequently, these decisions leave a bloodstained path and have hurt and traumatized countless other people, with Akiko Yosano and Osamu Dazai being prime victims of his abuse. Mori never takes emotions into full account, the epitome of logical thinking and strategy. That said, his intelligence allows him to manipulate and abuse many people by appealing to their needs and wants. Emotional manipulation and abuse have gotten Mori far in his motives at the cost of many lives, and by the time someone's realized they've been used, it's often too late for them to turn back. To Mori, a 'heart' only gets in the way of war, a belief leading to his irredeemable actions of using Yosano's ability and mentally breaking her during the Great War.
While a largely logical person who knows how to use words to his advantage, Mori is not above threats and intimidation typical of a cold, experienced mafioso. Age and gender do not matter to Mori, but when it comes to his grander schemes, he has a history of manipulating children, as their inexperience makes it that much easier to control them.
Mori's use of manners and polite conduct mostly attribute to maintaining a sense of control and power in any situation. For the most part, acting like a 'normal' person makes those around him less wary, as when he's in public and needs to avoid suspicion. Otherwise, it creates a certain distance between him and others, as if nothing they say ever fazes him. Yukichi Fukuzawa, his former ally, knows the depths of Mori's darkness, and he and Dazai are most often the ones to see straight through Mori's many pretenses. Once this facade disappears, Mori's intensity and willingness to do anything, even kill, become apparent and unsettling. His intensity is so much that Lucy Maud Montgomery is paralyzed with fear when Mori confronts her.
He does treat his subordinates with respect, valuing their skills and contributions to the organization and acknowledging their power. As such, even when facing a certain degree of insubordination, Mori is willing to overlook some mistakes so long as the outcome benefits the Port Mafia. This contributes to why he never punishes Ryunosuke Akutagawa's rash behavior. Even then, according to Mori, "effort is what matters, results come second", in the face of Akutagawa's previous failure of capturing Atsushi Nakajima.
This aligns with Mori's leeway towards the organization's most active, powerful members, allowing them a certain degree of 'freedom' and independence from his tight scrutiny. Generally, he usually only regularly speaks with his two most active executives, Kōyō Ozaki and Chūya Nakahara, the former of which often the first to criticize Mori's eccentrics in the form of dry banter. He and Kōyō converse the most often, and their partnership leads to Mori agreeing to spare Kyōka after she defects. As a result of the Port Mafia's prosperous control in Yokohama, Mori enjoys being held in high regard, either by respect, fear, or a mix of both, by his many subordinates.
Mori views the position of an organization's leader as both ruling them and being their slave. This belief drives Mori to his drastic and ruthless strategies. Furthermore, he is more than happy to use a common enemy to his advantage, as when revealing Naomi Tanizaki and Kirako Haruno's hideout to the Guild in hopes of using the Agency to take care of them and alleviate the Port Mafia's own workload. He uses the same tactic when leaking the orphans' location to André Gide, getting the orphans slaughtered and driving Odasaku to his own suicide mission. In both cases, Mori makes it so his direct word never reaches its intended target, leaving the power of his actions to force their hand instead. This manipulation and usage of knowledge is the core of Mori's decision-making and tactics, making him dangerous.
Losses and failure are Mori's worst enemies, such as a large number of subordinates and racketed collaborations dying during the Q incident, or Kōyō's being held at the Armed Detective Agency. Likewise, he detests "parasites" such as smugglers and the ilk, people he believes they suck Yokohama dry for their own gain and upset its delicate balance. Given Mori's respect for Soseki Natsume, Mori agreed to keep balance in Yokohama, and Mori takes this seriously. For as cruel and cold Mori is, it's unmistakable that he's vital to Yokohama's balance.
The only time Mori's acknowledged a major oversight in his decisions is when taking in and mentoring Dazai. He largely underestimated Dazai's own intelligence and shrewd, detached way of looking at the world. Ironically, their near-identical nature unsettled Mori. Nonetheless, Mori takes his mistake and molds it into a valuable discovery: a subordinate and right-hand that knew his way of rationalizing. Control over Dazai was crucial to Mori, considering Dazai's intelligence could one day be his undoing. This largely contributes to Mori's decision to use Sakunosuke Oda, a dear friend of Dazai, in eliminating Mimic and driving Dazai to defect.
Truly, the only person privy to Mori's less menacing and controlling side is Elise, even though there's incredible irony in the fact that even Elise's demanding, harsh attitude against him is something he intentionally configured. Mori has an obsessive nature towards Elise, irritating her with excessive trips to dress shops, tailors, and the like just to dress her up for his own amusement. Much to Elise's exasperation, even her harsh nature is something Mori enjoys. Given that Vita Sexualis manifests as a form of Mori's desires, it's unfortunate to note that he's a self-admitted pedophile, something both Elise and Fukuzawa have expressed great disgust in, the latter considering Vita Sexualis a disgusting and disturbing ability.
His ability, Vita Sexualis (ヰタ・セクスアリス, Wita Sekusuarisu?), is an ability that allows Mori to summon and 'configure' Elise's personality in any way he wills; he can make her levitate, chase after an enemy at high speed, and make her save him no matter what. He can summon Elise at will and make her attack his enemies with a barrage of large medical equipment.
Elise has shown her displeasure in Mori's ability in which she stated he always gets to "set the scene" and make all the decisions. Although Elise is powerful, Vita Sexualis requires strength and energy, thus when infected by Alexander Pushkin's virus, summoning Elise takes a toll on Elise. When Fukuzawa cut her down, she disappears. As Mori's ability, Elise doesn't die, but it takes time and energy for Mori to resummon her.
An important detail to note is that Elise used to be more subservient and borderline emotionless as Mori's ability. However, sometime after Yosano's mental breakdown, Mori intentionally configured Elise to mimic the young Yosano's defiant personality, showing a deep-seated obsession towards Yosano, of whom whose stubborn nature Mori frequently fawned over during the Great War.
- (To Dazai) "The Boss... died of a serious illness. His dying wish was to have me succeed him as the next Boss. And you were the witness, understood?"
- "The Detective Agency and the Port Mafia. If the government higher-ups catchword... that the heads of the two largest Ability organizations in Yokohama are having a secret meeting, they will definitely be foaming at the mouth."
- "Till next time, Dazai-kun. The invitation to resuming your duties as an Executive member in the Mafia still stands."
- There is a great gap between Mori's concept and the work chosen for his ability. Vita Sexualis deals more with the protagonist's 'discovery' of his own sexuality, but no paedophilic content involved. The only similar thing is that the love interest of The Dancing Girl, Elise, is roughly 16 or 17, whilst a protagonist is an adult man. As for his author counterpart, Mori was indeed a military surgeon who respected his logical and direct nature, being a respected figure in Japanese history and even being involved in criticisms of censorship, but the real Mori was never a pedophile.
- Being a strategist, he has read many works concerning game theory and war tactics and is an expert on the topic.
- Elise's name originates from the authors The Dancing Girl, where a Japanese man living in Europe means a young woman named Elise.
- Elise calls him "Rintaro", which is the author Mori Ōgai's real name. It is unknown if this is Mori's real name as well.
- The Silver Oracle he provides Odasaku with begins with the Latin phrase "Nihil admirari", translating as "Let nothing astonish you,".
- His preferred weapon is a scalpel.
- The author joined the military in order to study medicine, rising in rank to a head surgeon, reflecting the mafia boss' previous occupation as an underground doctor.
- In the anime, he appears next to Chūya in a building where a reception desk sports a logo resembling an M with the words "Mori Corporation" beneath. This is likely a shell company of the Port Mafia..
- Bungo Stray Dogs Volume 15
- Bungo Stray Dogs manga, volume 9, chapter 37
- Bungo Stray Dogs manga, volume 12, chapter 50
- Bungo Stray Dogs manga, volume 16, chapter 65
- Bungo Stray Dogs light novel, Osamu Dazai and the Dark Era, Chapter I, page 19
- Bungo Stray Dogs anime, episode 21