Masami is an adult man. He has medium length blonde hair and blue eyes. He wears the Amare Patriarcha Crucis nurse uniform, consisting of a Prussian blue shirt with black buttons on the right side, white pants, and white shoes. He has a pen and his ID card attached to the left breast pocket and medical supplies in the left waist pocket.
Masami is a 23 year old registered nurse who works at the Amare Patriarcha Crucis hospital. He is also Ritsuko Satomi's fiancé. From his soft demeanor and his refined features, Masami is a huge hit among the female staff; naturally, he's also extremely popular with the female patients as well. He took three years in nursing school and then followed it with two years of university training. He passed the medical licensing examination on his first try. Masami became acquainted with Ritsuko at the hospital and fostered a deeper relationship with her quickly after. 
Masami is mentioned only in passing through the text messages he sends to Ritsuko, in which she talks about him to Ayame Itou and Chiyomi Homura during most of the game.
At the final segments of the game, it turns out the attack on Ayame's group is perpetrated by Masami. After capturing everyone, he punches Ritsuko and lifts Ayame's shirt, revealing an empty hole in her chest while laughing maniacally.
Significant plot details end here.
Coincidentally another character named "Masami" was going to be in Corpse Party 2: Satsuki's Heart who was also described as "quick-tempered" but because the project was dropped, he never exists.