The male suspect denies committing the murder, according to what his lawyer, Anders Olsson, told news agency TT.
On Thursday night, the man, the woman and the victim were discovered in a car outside a fast-food restaurant in Säffle. Police were first notified of a quarrel in a solitary house on the outskirts of town. That is where the victim, 18, is believed to have been knifed.
The two suspects are reported to have been involved in a relationship at one time.