Here is the full list of all 40 Among Waifus 18+ achievements.
You've unlocked Girl 9
You've unlocked Symbol +
You've unlocked Symbol ?
You've unlocked Symbol !
You've unlocked Girl 8
You've unlocked Level Z
You've unlocked Level X
You've unlocked Level Y
You've unlocked Girl 7
You've unlocked Level W
You've unlocked Level V
You've unlocked Level U
You've unlocked Girl 6
You've unlocked Level T
You've unlocked Level S
You've unlocked Level R
You've unlocked Girl 5
You've unlocked Level Q
You've unlocked Level P
You've unlocked Level O
You've unlocked Girl 4
You've unlocked Level N
You've unlocked Level M
You've unlocked Level L
You've unlocked Girl 3
You've unlocked Level K
You've unlocked Level J
You've unlocked Level I
You've unlocked Girl 2
You've unlocked Level H
You've unlocked Level G
You've unlocked Level F
You've unlocked Girl 1
You've unlocked Level E
You've unlocked Level D
You've unlocked Level C
You've unlocked Level B
You've unlocked Level A
You've unlocked Girl 10
You've unlocked Symbol #