Doctor Tom is Erica's "Therapist". He has the power to send Erica back in time to correct regrets in her past. However, he himself is also a patient.
Erica first met Dr. Tom after having the worst day of her life and ending up in the hospital due to an allergic reaction. He meets het at the hospital, already knowing quite a bit about her. With little explanation, he hands her a card with an address and simply "Dr. Tom" as his name. Erica decides to meet with the strange doctor, and he has her write out a list of all the regrets she's ever had in her life. He reads out one at random. Erica talks about what happened in the regret and then explains what she would do if she could do it over. Without any warning, she is suddenly back in time reliving the regret with her memory intact, allowing her to change the past. While in the past, Doctor Tom will periodically come by to check up on her and give her advice. When Erica has learned a certain lesson, she returns to the present and incorporates this change into her life, which for the most part improves her life. This pattern is followed for most sessions Erica and Doctor Tom have together.
In the season 2 primere, we learn that Doctor Tom was a normal man with a normal life before becoming a Therapist. He was married and had a daughter. Dr. Tom sees much of his daughter in Erica, and will sometimes project feelings onto her in the same manner. Doctor Tom committed suicide, which somehow lead to him becoming a therapist. When Dr. Tom's life is explored, Dr. Angela is the guide; she seems to be Dr. Tom's own Therapist.