Erin Davis is a possible love interest for Stephen Loyal, the main character for the story "Midnight Masquerade" that is included in The Sims Pet Stories. She is a good friend of Stephen's cousin Celeste Girard and is also the person who delivers Celeste's cat, Diva, to be taken care of by Stephen. Originally, Erin was assigned to take care of Diva, but declined because Buster, her dog, "keeps using her as a pillow". She is implied to be a self-employed artist, but also mentions that she used to be a practicing veterinarian, which is why she is so good with animals.
Gordon is then framed by Stephen Loyal at the Masquerade and arrested. Stephen is then announced King of the Masquerade. It is up to the player to choose his queen, either Erin or Rachel Ware, which makes her playable at the end of the story.
Notes
There is conflicting information on if there is a glitch if the player chooses Erin as the Queen. See both links in the references section.[1][2]