Mariana cares for him a lot, and she knows him better than anyone. She also looks rather annoyed when she sees him flirting with others.
http://i.imgur.com/SqpF1EA.png
http://i.imgur.com/2N94YHz.png
And sometimes she'll lead his attention away from those others.
This night at the bluffs, she even let him put his arm around her while they were gazing into the fire.
http://i.imgur.com/8qVhfBv.png
http://i.imgur.com/sZSzWqs.png
As much as Vetle is trying not to get his hopes up, he can't help but notice her poorly concealed affections for him.