I know the whole series hinges on the concept (and most of its comedy) of unrequited love, but I always felt Yakumo's story to be so tragic. She is surrounded by guys only wanting to be with her and she finally meets Harima, who treats her as a friend and confidant, and as a result, she falls in love with him. Yakumo ultimately gives up on Harima because of his feelings for her sister, but Tenma does this round-about rejection on Harima that completely (unintentionally) wastes Yakumo's sacrifice. That will always break my heart