No, Korra is not Bolin's. She isnt even Mako's because she is owned by no one. She owns herself but she CHOOSES to give herself to Mako. It would be pretty selfish for Bolin to claim Korra as his own or try to pursue a relationship with her when she doesnt like him in a romantic way, doesnt want a relationship with him, is in a relationship with his brother who she loves and who loves her back. Just sayin.