Linnea Opjer is Nikolai's half-sister.
King of Scars
In the scene where Zoya is reading off candidates for Nikolai to potentially marry to, Linnea's name comes up. Zoya describes her to be young, charming, and brilliant in Mathmatics; Nikolai shrugs it off and says "I expect nothing less from my half sister". Zoya is surprised as Nikolai's parentage is questioned since he was born.
Rule of Wolves
Linnea Opjer is a student of the University of Ketterdam studying engineering. When Magnus goes to see Nikolai at Lazlayon, he tells his son that because of Fjerdian law, she can not inherit his money. Magnus asks that Nikolai take care of his half-sister.
Linnea is invited to Zoya Nazyalensky's coronation. Nikolai introduces Inej Ghafa to her at the after party.
The name Linnea is of Scandinavian origin which means "lime or linden tree".