The even/odd costs are slightly annoying, but that's part of a good CYOA img -- forcing you to ''choose' something, and choose to dismiss other things.
As a resident furry, I really like Wyn, but there are such posibilities with Lunara! So, here's how I spend my coins:
Lunara: unlocking transformation, invisibility, body swap
Wyn, and her polymorph skill
Synzetica, because I have exactly one coin left and she's the least likely to irritate me.
So, here's my plan: Lunara changes to a beautiful centauress. Then changes place with Synzetica, leaving Synz the beautiful centauress. Wyn makes herself much more … equine and at least some of the time, as per my 4th pic, myself too.
Later, Lunara can change the body she's wearing to a well endowed, lactating cowgirl, and Wyn can help me relieve the real Synzetica's udder discomfort.
During the day I can just ride around on Wyn-The-Centaur, like in pic #3, and Lunara can make the three of us invisible so I can play (with) lady godiva and actually get away with it.