Sawada Shin is eighteen years old, and for the first time in years, he feels like a kid. Teenage hormones have been forcefully reintroducing themselves to Shin over the last several weeks. At his sourest moments, Shin suspects that this is payback for laughing at his friends' romantic misadventures.
Yankumi looks ridiculous.
Shin knows that he's in trouble because he still thinks that she's adorable. The eighteen year old boy Shin likes to pretend he isn't is busy sneaking looks at Yankumi's legs. Everyone is treating Yankumi like a joke, and Shin wonders if he's the only one who notices how short her skirt is. Her knees are exposed, thighs bare--and Shin is practically ogling his teacher in public.
Shin isn't just thinking about sex. He's not even primarily thinking about sex. Sex would be easy, and excusable--Shin is eighteen, after all. He wants to see her smiling and happy, and free to be an idiot as often as she wants. Thinking like that is dangerous. It's proof, as if Shin needs more, that Yankumi is under his skin.
At eighteen, Shin feels like a boy suffering through his first crush.
At eighteen, Shin is already older than he ought to be: whatever this is, it is no simple crush.