Ruka is the best girl,
You guys keep arguing about how he blackmailed into going out with him but its not like she has done with a bad intention.
She is not totally undesirable for the mc, the mc find her cute too. And i don't really see a problem with her personality.
If he were to move on with the other heroines, she would be the perfect girl for him, her frank and agtessive personalitt suits his beta personality better than the other heroines.