Hahh... can't even read my feelings.
Kai Nagase is the son of the main antagonist in Sukisyo. Like his father, he is also stepping up to becoming a professor. In the game White Flower and Mou Matte, Nagase is also a substitute teacher, the chemistry lab advisor, and the elite member if the Be-Reave primary, the school's advanced Chemistry department. He is voiced by Ryoutarou Okiayu.
Nagase has short teal-green hair with long, lower chest length side locks. Like his father, he also has violet-blue eyes and wears a a pair of glasses. Nagase is almost always wearing a long, white lab coat and a dark purple turlteneck. Nagase is known for being expressionless, but the creators of Sukisyo describe how Nagase actually does have a range of facial expressions. He has eye-length bangs that are thicker in the middle, thinner at the sides. He is also probably one of the more mature characters in the Sukisyo cast.
Not much is revealed about Nagase's past in the anime. He is seen as a child taunting Sora in the lab shortly after being kidnapped. "Father said... no one would give a damn if you two disappeared."
In the White Flower novel series, however, it reveals a world of difference. It goes in to detail stating that Nagase Kai is not "like other people", derived from the fact that Nagase was a test tube child who was originally going to be Aizawa's experiment. It states that while Aizawa had Shinichirou in his lab years ago, Aizawa used his and Shinichirou's DNA to create Nagase, and one drawing in particular from official material has a picture of Nagase and Minato comparing the two.
As a child, Nagase was quiet, never one to talk or speak or socialize. Gaku would frequently visit the lab as a child and the two played for hours. While Nagase remembers this and cherishes this memory, Gaku cannot remember it at all. (This can be seen in a flashback chapter of Sukisyo's OFFICIAL manga "After Stories", as well as in the novels) Nagase has always truly loved Gaku since his childhood, but never openly admitted it, mostly to protect Gaku from his father Aizawa. Nagase was always over protective over Gaku and always kept Gaku's interest in heart. Aizawa had gotten attatched to Nagase, and with the help of Nanami's departure, chose Sora and Sunao instead.
Nagase grew up to resemble his father, much to his unacceptance. He is witty, quiet, and even deemed as socially awkward and scary from his peers. Nagase rarely speaks, but like his father, speaks in a more flourished vocabulary while most of the other casts uses a heavy Tokyo accent. Nagase grew up to study and advance in science and chemistry as well, and even experiments and collabs with his father. Both have many thesis printed in scientific magazines.
Nagase may be in love with Gaku, but that doesn't stop Kirito Nakahara and Ren Shiina from getting in the way. While Kirito may have a crush on Nagase, Nagase does not have the same emotions towards Kirito, and Kirito constantly attacks Shiina and Gaku in a sad attempt to gain Nagase's attention. Ren Shiina admits in Mou Matte and White Flower that he has a crush on Nagase Kai, but is also in love with Gaku Ichikawa. While Nagase disdains Shiina, he accepts this, and the three start a three-way relationship.
Nagase also becomes friends with Sora Hashiba, the two seem to have a love-hate friendship on the games, audio drama, and novels. In the anime, Nagase dies along with his father in a fire caused by Sunao shortly after saving Sora and Sunao, redeeming himself.
Kano is very attached to Nagase and Gaku, always clinging to his coat and bothering Nagase to play with him. Gaku thinks this is cute, while Nagase gets very anooyed with it. He addresses Nagase by "Oni-Chan" and seems to never want to leave his side. Ayano often jokes saying it's almost as if Kano is their child instead.
Nagase is a quiet young man who seems to over work himself. He is very clever and sometimes critical with his thinking. Many students fear him, he is however really popular with the chemistry department. While he may seem emotionless to a certain degree, he normally can express himself, at least as much he can, around Gaku. Nagase is all around a good guy, but because of his father, is often avoided and misinterpreted.