There's two main aspects of Koikatu. The first is the main event, the game itself, which includes the character creator, and then the posing studio.
Koikatu itself is premised on you being the sole guy who transfers to an all-girls academy where your Aunt is the principal. She charges you with being the romantic interest of every girl at the school, and you start a harem club. Gameplay consists of running around between classes, and chatting with girls in typical dating sim fashion, inviting them to your club, and managing them as your own personal sex dolls. There's some basic character drama from some pre-set characters like teachers and class presidents and such.
The main meat of the whole thing is the character generator. If you've played a wrestling game's custom character generator, it's around that in-depth. The options are vast and varied, with community contributions expanding it several fold. Unfortunately, fundamental differences are limited since you can't do much with the underlying base model and thus every character becomes samey. This is pretty par for the course for Illusion games. The organization of the options is terrible, though. I felt like I was getting carpal tunnel scrolling through the thousands of clothing items.
As for options beyond the aesthetic, there's a pretty sparse level of personality traits, which mostly consist of which pre-canned responses you'll be reading over and over, and shallow interests. Does this girl like animals? Is she fashionable? Yes or no, and that's far as it goes.
Attached to Koikatu is the a bit of posing software, and here's where the true limitations of what's possible come spilling out like so much dirty laundry hidden just under the bedspread. Using the scene software is painful. It is clunky, obtuse, finicky and primitive. From the GUI to selecting bones/articulation points, it feels like something from 20 years ago. Source Filmmaker came out in 2012 and it feels years ahead of this thing, much less something modern like Blender. If the software made it easy to export its library of assets, I'd recommend never touching this thing.
Overall, if you want to play dress up with some dolls to look like your favorite characters, this is fun for an afternoon.