Here is the full list of all 20 Kaiju Princess achievements.
Obtain the Bikini
Reach Mouth Accomplishment Lv.3
Obtain the Magical Teen Outfit
Reach Breast Accomplishment Lv.3
Reach Clit Accomplishment Lv.3
Reach Vagina Accomplishment Lv.3
Kisses x1000
Reach Orgasm x4
Reach Orgasm x8
Obtain the Maid Outfit
Obtain Black Cat Cape Doll
Reach Orgasm x2
Obtain Tilapia Doll
Obtain the Competition Swimsuit
Obtain Festival Fan Doll
Cum with Me
Obtain Popsicle Kaiju Doll
Obtain the Witch Outfit
First Quota Reached
First Time