Initially, Tamao is the girl who Ranmaru's parents want their son to marry. Ranmaru did not want to have any fiancées nor get married since he is not into a committed relationship and is a ladies man, so when he first meet Tamao, she was the complete opposite of him and wants a relationship. But despite his unusual self, Tamao finds him intriguing and slowly falls for him. But Tamao admits that she really likes Ranmaru, even though Ranmaru tried everything he can to be "hated" by her. Tamao also confesses to Ranmaru and openly admits that even though Ranmaru doesn't reciprocate the romantic feelings she has for him, she still likes him and will not give up on him easily, something that Ranmaru actually found attractive when she admits that and that's when he began to have feelings for Tamao.
Later in the anime, Ranmaru says that he always think about Tamao but Tamao is not his "taste" since she is about commitment, hinting that he is starting to finally realize his romantic feelings for Tamao.
In the manga, Ranmaru is starting to be aware of his feelings and tried to initiate a kiss with Tamao in the end of Volume 19. He is also seen blushing while carrying the hurt Tamao when they went onto an outing in Volume 19.
Ranmaru's parents also both like Tamao because they've always wanted to have a cute daughter and think she could be a good influence on their son, convinced she could give Ranmaru stability and curb his often unhealthy womanizing tendencies that sometimes got him in trouble and broke his heart and settle down with a beautiful girl he can love.
While Ranmaru states he doesn't want to get married and wanted Tamao to hate him, in an episode when Tamao saw him with another woman and she ran off crying it devastated him knowing he hurt the girl he does love. This states that he probably doesn't want to marry Tamao because his womanizing nature could hurt her and have her be in a disappointing marriage where they would both be unhappy. But soon he starts to win her back when he realizes he wanted her and only her.