Here is a logic to the ritual step.
Ritual steps:
1. Get the skeleton to gain the ritual background.
Then the rest is basically do the ritual by using the pumpkin head on these 4 girls to unlock the rest.
2. Do the ritual with the sheep. (You get Cult of the Lamb or Demon Sheep.)
3. Do the ritual with the bee. (You get Hornet)
4. Do the ritual with the endergirl. (You get Mal0)
5. Do the ritual with the pumpkin. (You get Spice, which is the 6th of the ritual steps.)
Thats all there is to it.
So the 6 characters from the achievement is:
Skeleton, Demon Sheep, Hornet, Mal0, Pumpkin, Spice.