Theresa Diaz

Season 1

Theresa is a childhood friend of Ryan Atwood, and the two have engaged in an "off and on" relationship, which came to a halt upon Ryan's departure from Chino. Over the Thanksgiving holiday, Ryan briefly returned to Chino to pay off Trey's debts, at the request of a still incarcerated Trey. Ryan went to Theresa's house to pick up a car from her brother that would later be used to pay these debts. During this time, Theresa and Ryan each exchanged apologies about the lack of communication during Ryan's absence, most notably Ryan's failure to inform Theresa of his exodus from Chino.

Soon after Theresa and Ryan met again, when Theresa began working in Newport for unexplained reasons. Eventually, Theresa revealed that she and Eddie (a friend of Trey's from Chino) had become engaged. Soon after the engagement, Theresa developed cold feet (most likely due to the domestic violence she suffered at Eddie's hands). A short time before this, Marissa Cooper and Ryan had broken up over recent complications involving Oliver Trask. Naturally, Theresa and Ryan began a relationship soon after. Eddie persisted in his attempts to win Theresa back, even showing up at Harbor High School to confront Ryan about his suspected relationship with Theresa. This finally culminated in Eddie making an appearance at the congratulatory party for Caleb Nichol's being named Man of the Year. Eddie and Ryan came to blows, with Eddie being dragged away from the party and Ryan drenched in the pool. Theresa realized that she needed to resolve these issues, and left Newport to return to Chino and Eddie.

However, Eddie continued his abusive behaviors, and Theresa finally left him. She returned to Newport, seeking Sandy Cohen's legal advice. While Seth Cohen, Ryan, Sandy, Jimmy Cooper and Caleb are in Las Vegas for a "bachelor's party", Marissa discovered that Theresa is pregnant, with the father as yet to be named. For a time, Theresa reluctantly considered an abortion, but Kirsten Cohen informed her that she still had a choice in the matter. Theresa decided to have the baby, and went back to Chino. Ryan, in an attempt to ensure that the baby has a better childhood than he had, left his newfound life in Newport to help Theresa raise her (and possibly his) baby.

Season 2

Theresa sensed Ryan's reluctance and doubt, so she deceived Ryan into believing that she had a miscarriage, allowing Ryan to return to Newport, and the Cohen's. Theresa does not appear again until the end of season two, in The O.Sea, where she convinces Ryan to trust Marissa's explanation about what happened between her and Trey. We learn at the end of this encounter that she has the baby, is living with her mother, who informs us that the baby "looks more and more like his daddy every day".

Season 3

Theresa drops off the map again until the end of season three, in which Kirsten spots her with her baby at the airport. She informs Sandy of this turn of events, who tells her not to tell Ryan, as he will no doubt interrupt his college visit at Berkeley. She does tell Ryan, however, who lives up to Sandy's expectations, and returns home. Theresa comes to visit him, and informs him that the baby is in fact not his, but Eddie's. (However, it's strongly suggested that the baby is in fact Ryan's since it is seen with all light features like Ryan; Theresa and Eddie are both Hispanic). The two spark another of their "relationships" for a short time, but Ryan's fight with Kevin Volchok convinces her that Ryan has not yet matured enough for a relationship with her, and fears the influence Ryan might have on her baby.