"You want to know what death is? I'll tell you. Death is the loss of life. Despite everything doctos like me attempt... a patient's life can still fall through our fingers. You think death lies in the apex of science? Anyone with such little regard for life will die by my hand."
Yosano is a young woman who takes care of her appearance. She has black hair with straight bangs cut into a bob that reaches past her chin, and magenta eyes.
She wears a white button-up blouse with puffy sleeves that end in the middle of her forearm. Her shirt is tucked into a plain black skirt that flares out above the knee. She wears gloves that reach past her wrist, a black tie, and black tights. Yosano wears red high heels and a golden butterfly hairpin on the left side of her hair.
Yosano is a lady who carries herself with utmost grace and dignity. She prides herself on being both a strong woman and a capable doctor, which is why she does not take too kindly to those who belittle her in both respects. She appears to be a feminist who believes now is the age of equal rights for men and women. She almost always depicted as calm and composed, but when angered, she is quite the force to contend with, often sadistic in her retaliation.
She's prone to prominent sadistic and aggressive tendencies, which is more so magnified by her ability's conditions. As a result, most members from the Agency flee when in her presence and as much as possible, avoid getting injured even in the slightest to circumvent her unique brand of “treatment”.
As the sole doctor of the Detective Agency, she highly values human life and is significantly averse to meaningless deaths and killings. She will violently admonish those who believe otherwise and is very keen on doing so without much hesitation.
Out of all of the Armed Detective Agency, she seems to be the closest to Ranpo Edogawa, as he was the one who introduced her to the Agency.
On a side note, she appears to love shopping to a great extent, and when not working is often showing buying large amounts of clothes. She often makes one of the other agents come with her, having them carry her several purchases.
Yosano's strong personality stems even from her childhood, as she's shown to show little concern for soldiers' well-being during the Great War. This is likely because she believed her ability made their injuries "no big deal", and as soldiers, she expects them to toughen up. Always holding herself up with great self-respect, she butts heads with Mori, despising his possessive nature towards her.
However, as the war rages on, Yosano realizes her ability can only heal physically, not mentally or emotionally, watching more and more men break under the trauma of war and her ability's conditions. The situation breaks her down, but Mori forces her to continue to the point where she shuts down emotionally, a shell of herself. It isn't until after Ranpo Edogawa and Yukichi Fukuzawa save her from Mori's influence that Yosano starts to recover. To this day, she detests Mori.
Yosano has a rare ability among ability users, Thou Shalt Not Die (君死給勿, Kimi Shinitamō Koto Nakare?), which allows her to heal external wounds. However, to heal someone they must first be "half-dead"; meaning they must have fatally serious injuries before she can use her ability. Due to this, her ability is inconvenient for healing minimal wounds since she needs to fatally injure that person first.
Upon activating her ability, several butterflies come out from her body and land on the surroundings. When touched by them, the injured will have their injuries healed instantly.
She is also quite skilled in close combat, showing proficiency in wielding a large cleaver that she occasionally uses.
- "You want to know what death is? I'll tell you. Death is the loss of life. Despite everything doctors like me attempt... a patient's life can still fall through our fingers. You think death lies in the apex of science? Anyone with such little regard for life will die by my hand." (to Motojirō Kajii) 
- She made an appearance along with several other characters in the mobile game "Kimito Lead Puzzle 18" as a limited-time character.
- Yosano is named after a feminist pacifist poet whose work was considered controversial in her time for portraying and encouraging female sexuality. Her ability is likewise named after one of the real Yosano Akiko's poems, which was addressed to her brother and published during the height of the Russo-Japanese war. It was subsequently made into a song and sung in protest to the war.
- She says the obituary is the most interesting section of the newspaper, as it judges people "the most fairly," to which Dazai agrees.
- Yosano reveals that she and Mori had a connection long ago. This also applies to their real-life counterparts, as Mori assisted her in finding travels outside Japan; Yosano was a member of one of Mori's poetry groups and also asked him to name her twin daughters.