What does Yako offer here that couldn't have already been fulfilled by Umimi?
I will add that she also tickles long hair and small breast fetishes, as out of them girls her hair is the longest - the other two had such hair back in time now they cut them short, and her tits are the smallest, thou not petanko near flat levels - but it still makes her fine while the other two are kinda going past the edge of "to big" for petan enjoyers.
EDIT: Also after reading the chapter finally (I tend to go comments first) - Yako best girl no doubts. Each time she has her time to shine, she really does shine. The other two are either doing manipulative, secretive, dumb shit, or are being tried to NTR by other dudes like Shizu is on her way to get drug groped by the manager. Like MC should date Yako and change schools with her to get away from the other 3 harem members including gay Takt, who really only make his life a miserable confusing hell. Yako seem to just like him and tries to make his life better, cheer him up, etc.