Here is the full list of all 12 Summer In Trigue achievements.
Unlock Grace romance CG
Unlock Grace pin-up CG
Unlock Alex pin-up CG
Unlock Alex romance CG
Unlock Mia romance CG
Unlock Shiori romance CG
Unlock Liz romance CG
Unlock Mia pin-up CG
Unlock Shiori pin-up CG
Unlock Liz pin-up CG
Unlock all romance CGs
Unlock all pinup CGs