Naoko is the protagonist's adopted big sister. She has an extremely straightforward personality, and she tends to spoil the protagonist rotten while thinking she's doing the reverse.
Unfortunately, her path, like Kirsti's, is lacking in the kind of ending drama that tended to characterize Minato's two big hits. I don't know whether they did it deliberately or not, but the path left me feeling unsatisfied, despite the fact that the VN as a whole is at least moderately amusing.
The harem path is basically this side-route where the protagonist doesn't choose Naoko or Kirsti and ends up in H-situations with about half the cast of females. There is no real content to it, except for somewhat forced situations that end up in the protagonist having sex with one or another of the girls. To be honest, after playing this, I almost thought the VN as a whole was a nukige.