Josephine is not a flirt, really.

Part of being your matesprit is learning how to swim. You don't know how Karkat got this far without being able to, but before the end of the night, he's going to learn if it krills you.

Kaz recruits Wylan to help infiltrate the Ice Court. Wylan is under the impression that Jesper is flirting with him—until Jesper brings up his lost true love.