Where did you get that idea that Aiko is going to join the harem? True, she has a thing for Hajime, but all she is is a side character attracted to main guy. She hasn't been accepted by Hajime or Yue yet, nor has any development between her and Hajime even after the latest volume of the LN, and I highly doubt there will be. And Hajime has not had sex with anyone other than Yue and Shea at that point too. Yeah, he has accepted the girls as companions he cares deeply for, but that's it. Maybe eventually he will go at it with the other three in the future, but he hasn't done so yet.