Miko is playing the long game here because if you get Suisei, you will also get her sister, Anemachi, who can't live far from Sui. And Anemachi is not just a useless add on either, here is Aqua raving about how absolutely beautiful Anemachi is:
https://youtu.be/D-7kVQZsBGIAnd Kanatan said before how Anemachi and Suisei look very similar to each other. So in short, by successfully wooing a cute girl, Miko will also successfully woo another pretty girl with bigger tits.
