I gave him 6 Swadian Knights for the quest, and it boosted him to my friends list, but he still refuses because I am not a vassal.

I had about 18 renown with her father and he refused based on the fact that I am not a Swadian vassal. She is wearing a fur coat so I assume she is tomboy type rather than romantic. I simply walked up to her and told her I like her and I gained 17 renown straight away a good sign? I won a tournament and then dedicated it to her for another 4 points. The only one that really matters from a meta perspective is the defection. It also allows you to convince routed enemies to become your prisoners.

Her father doesn't have a fief of his own but her uncle has Uxkhal and between them her relatives own half of Swadia. For Persuasion, it helps you marry earlier, helps you hire mercenaries cheaper, and helps you convince lords to defect to your kingdom. I chose Lady Sonadel as I am currently Mercenary for Swadia and she has many male relatives and she is the least ugly in her family. Last night I made it my goal to find a wife in the game as it is something I haven't done yet.

Marriage mount and blade warband