Theresa is the oldest of the two Lopez-Fitzgerald daughters, and doesn't remember her absentee father very well, who disappeared when she was a toddler. She is very proud of her mixed heritage (half-Mexican, half-Irish), and even more proud of her long dark hair. She's been obsessing over Ethan Winthrop and what it would be like to marry money for years, to the point where she has the walls of her room plastered with pictures of the Crane heir. Her obsession with Ethan has led to a number of "accidents" where she dumped various disgusting things on him, a completely irrational hatred of his girlfriend Gwen, and stalking the young Crane heir. She almost gave up her delusions until a gypsy fortune teller told her that she would one day be married to a Crane and living in the Crane mansion. Theresa assumes that the Crane in question is Ethan, and that they will have a long and happy marriage. Her best friend is Whitney Russell, and they enjoy sharing their dreams -- Theresa's of being a high-priced fashion designer, and Whitney's of being an internationally reknowned tennis star.

After Theresa was outted as being Ethans stalker, she temporarily left her job at the Crane mansion on Ethans insistance. Later, Whitney "explained" the problems to him, and the two agreed to put aside past issues and work together. A chance encounter at a mountain cabin (engineered by Ethans father) convinced Theresa yet again that Ethan and she are meant to be together. Despite Ethans continuing claims of love for Gwen, Theresa refuses to let go, and is doing everything she can to get the man she wants. Ethans long-time love Gwen has gotten very suspicious lately, and Theresa has "invented" a boyfriend that she loves very much. Gwen almost uncovered her secret, until her "boyfriend" magically appeared just in the nick of time to save her from utter humiliation.

Unfortunately for Gwen, Ethan continued to buy into the goody-two-shoes act. It went so far that they were cheating on Gwen behind her back up until the night before the wedding. Less than twelve hours before the wedding, Ethan called a halt, telling Gwen he loved Theresa too. He wobbled back and forth between the two, but on Christmas Eve, he went to Church with one fiance, and came home with two.

During the same period, Theresa found documents that proved Ethan was actually Sam Bennett's son. She scanned them into her computer, and planned on using them to break Ethan and Gwen up if he chose Gwen over her (by blackmailing Ivy), even though her mother continuously told her that what she was doing was wrong. Her intention was to prove to Ethan that since his father wasn't a member of the upper class, he belonged with her and not Gwen. When Ethan proposed to her, she decided she didn't need her blackmail anymore, and erased it off her computer (at her mother's insistence).

She didn't realise that Rebecca Hotchkiss, who was still angry with Theresa for the way she manipulated Gwen and Ethan, had seen the documents. She sent them to the tabloid from Theresa's computer, and it all came crashing down at Ethan and Theresa's engagement party. Ethan has called a halt to their engagement for awhile so he can get to know himself. He pulled things back together, and the two went on with their wedding plans...until Rebecca and Gwen told the tabloid that she had sent the news!

The wedding was halted, and Theresa decided that the only way to get Ethan was to go to Bermuda and convince Julian to reinstate him as the Crane heir. Her plan didn't work though...Theresa and Julian both had too much to drink, and wound up married! Theresa's only real consolation is that she slept with Ethan before sleeping with Julian...and that by being married to Julian, she could have some claim over the Crane money. She's already asked for money once, and now she's demanding that, before she'll sign the annulment papers, Julian has to get the fax proving their marriage from Ivy, and force Ivy to never reveal that she and Julian married!

Unfortunately for Theresa, the truth came out...and Theresa learned she was pregnant! Because she was on protection when she slept with Ethan, the assumption is that the baby belongs to Julian Crane. Theresa isn't willing to risk Ethan after all her scheming...her plan is to abort the pregnancy. Despite both Pilar and Whitney begging her not to, Theresa is all set to go through with her plan. She might encounter a hitch, though, when her pregnancy starts to show, and Ethan starts to notice. When tabloid reporter Mort Chandler reveals her pregnancy, Ethan is shocked!