Kotaro Yui is a 27-year-old male with pink eyes and pink hair, which he typically wears in a ponytail. Standing 180 cm tall and born on June 1, he is a Gemini.

He works as a genius pharmacologist, deeply obsessed with studying immune-to-medicine properties DNA. This professional focus has led others to refer to him as "The Weirdo of The Narcotics" due to his eccentric words and personality. Despite his unconventional demeanor and extreme way of speaking, he is regarded as the manliest guy on the narcotics squad, a trait not immediately evident from his appearance.

When questioned about being treated as a weirdo, he expressed indifference, noting that such perceptions make it easier for him to do various things. His professional fascination is so intense that he stated if the Immune to Medicinal Properties were a person, he would chase after and live with that individual without hesitation. Outside of work, his personal routine includes feeding his mouse and reading theses while drinking a glass of wine at home.

He has lived in England before, which contributes to his background. His graduation thesis was titled "Flexibility of the Paraquat Poisoned Patients and Machine Model of..." though it remains unfinished in available records.