Okay, this was such a drag but I got the final Harem ending.
For anybody who is having a hard time, you have to do all 3 routes and see both of the endings of each routes.
So that means you have to do the solo ending + the ending of going through the Christmas Party for each girl.
Once you have seen all 6 endings (2 from each girl), keep the last one for a Christmas Party, and you'll see the Harem Ending.
Hope this helps someone, it was such a drag for me, since I was only missing 1 out of the 6.
