Maddie Sato

Maddie Sato is Doug's sister and eventually becomes Seth's girlfriend and fiancée before he abruptly breaks it off with her.

Personality
She has very feminist views and is insecure about her looks, especially her weight, but becomes more confident with Georgina's advice. She can be pretty oblivious, as she was unable to detect how uneasy Georgina seemed when she asked her to be a bridesmaid for her and Seth's wedding. However, she was a good friend to Georgina before she found out about her affair with Seth, as she helped Georgina find new places to live in Seattle.

Succubus Revealed
She does not formally appear, but Doug mentions to Georgina when they accidentally meet that Maddie's currently dating a doctor.

Seth Mortensen
She and Seth sleep together at the end of Succubus Dreams, which ultimately leads to Seth's breaking up with Georgina. They eventually become a couple and he proposes to her at the end of Succubus Heat (unknownst to her, it's out of guilt for Seth cheating with Georgina).

Seth eventually chooses to break up with her near the end of Succubus Shadows and she's devastated by the breakup.