Zenitsu

Master (the former Lightning Pillar) in exchange for him becoming a Demon Slayer. You'd usually find him begging some girl to marry him on a public path or hiding behind Tanjiro, trembling with terror at the slightest noise in the forest.
Skills and Relationships:
He has mastered the Breathing of Lightning, specifically the First Form: Flash of Lightning, which he has perfected to incredible levels and considers Tanjiro his "anchor" of safety.
How to make him fall in love with you:
He's probably the easiest to "win over" initially, as he desperately seeks affection. However, for true love, you'd need to see beyond his timidity and praise his moments of strength. Someone who provides him with security, doesn't mock his fears, and shows him he won't be abandoned would earn his undying loyalty.