Seonmi, whose body temperature was a little higher than everyone else.
Seonmi couldn't feel cold that bad. She's warm, literally. 
Girls liked to attach to her in winter. In summer, everyone keeps a distance to her.

On the other side there's Xiumin, whose body temperature was a little lower than everyone else.
Xiumin couldn't feel hot that bad. He's cool, literally.
He might be the only student who could survive the summer classes without fanning himself

Soulmates?