Astrid is an alchemist that lives in a cottage in the forest. She is responsible for creating several special potions.
You can only get the Priapus Draft, which turns your slave into a dickgirl, from her when you meet. If you are trying to get Nymph's Tears for Lady Farun, she will tell you about the Private Tutor. She will also create an antidote for Rei's addiction.
If drink the Priapus Draft 3 times, your slave's transformation will become semi-permanent. Astrid will offer an antidote if you come back 2 weeks later. If you refuse the antidote, your slave's transformation becomes permanent and you will enable the Dickgirl Ending.
Unless you start out in the Forest Home, her cottage location will not show up on your map when you Take a Walk. There is a random chance that you will meet her when you visit the forest. After get any slave to drink the Priapus Draft 3 times, her cottage location will be unlocked and accessible for your current all subsequent slaves.