Kosame is a fair-skinned girl with shoulder-length silver hair and violet eyes. She has a small bow on the left side of her hair that is colored mint and purple, on her face she wears a eyepatch which covers her left eye; her regular outfit is her school uniform, which is a pink long-sleeved sweater a white and green sailor-fuku with a green bow, a yellow and white plaid skirt and brown loafers, She also has a choker with a heart-shaped charm.
Despite not showing much expression on her face, Kosame is a calm, soft-spoken and friendly girl around people who does not hesitate to help her close friends. She becomes very nervous and loses her sanity if she does not take her pills. She is nicknamed 'Mental Girl' by Rina Shioi because of her sudden insane personality that makes her obsessively want to cut herself and needs to take medication to suppress it.
Kosame became a Magical Girl after she was diagnosed with terminal cancer, giving her only 6 months of life at best.
Kosame's ability is cutting herself with her stick which she can use her blood to heal her comrades if needed. However, she is unable to prolong the lifespan of others. She receives her stick from Hachi.
- Her hobbies include playing MMORPGs, sleeping, SNS, watching anime, online shopping, and collecting stuffed animals.
- Kosame said to Aya Asagiri, Tsuyuno Yatsumura, and Rina Shioi that she is fine with dying anytime since she will die anyway because of her cancer. But she still prepares for the Tempest but doesn't reveal the reasons why.
- Her character design and personality are strongly influenced by menhera, a Japanese fashion subculture based around the theme of 'sickly cute'.
- Her name 'Kosame' translates to 'Light rain'.
- She loves things that are supernatural and sweets.
- She dislikes human garbage and children.
- Her birthday is on November 6th 
- Along with Kiyoharu Suirenji, Kosame's horoscope is Scorpio.