You can clear it by following the main quest, and the costume is just a temporary one used a few times during said quest. If you want to wear the costume outside of the mission you can do so by accessing the costumes variable using something like Mtool by going to variables and changing the costume value from 0 to the following:
1: Maid Outfit
2, 6, 7: Lewd variations of main outfit
3, 4, and 5: Tavern Outfits
8: Pink dress
9: Powered Outfit
10: Brothel Outfit
Likewise you may change her hairstyle as well, which function independently from the outfit. So if you like the twintails from pink dress with one of the tavern outfits, for example, you can easily mix and match.
