Gwenyth is a minor Ghoulie in Grabbed By the Ghoulies. You cannot fight this ghastly green-haired specter in the game. However, she can be 

Lady ghost.jpg

seen in several portraits throughout the mansion, most notably in the Grand Hallway.

It is unknown what her role was in the game, but the E3 2001 Grabbed By the Ghoulies Video that is unlocked by completing the game implies that she had a major part.

In random situations if the player is near the window, Gwenyth will appear outside the window and scares the player.

According to Rare artist Ed Bryan, Gwenyth's full name was Gwenyth Von Ghoul, the Baron's possible wife. It is implied that if the Baron was happily married, losing his wife may have been the reason for his descent into insanity, as well as his interest in the supernatural and the world beyond.