Today the other woman took the stand. The big question here -- was gypsy willis just martin mcneill's mistress or was she the reason he allegedly killed his wife? Here's abc's adidi roy with the... See More
Today the other woman took the stand. The big question here -- was gypsy willis just martin mcneill's mistress or was she the reason he allegedly killed his wife? Here's abc's adidi roy with the latest in "crime and punishment" Reporter: When she walked into the utah courthouse, gypsy willis was center of attention. Miss willis, if you will come forward to the clerk's desk. Reporter: And she didn't shrink from the cameras when it was team to take the stand. My name is gypsy willis. Reporter: And give some of the most anticipated testimony of the martin mcneill trial coming face to face with the man she says was once her lover. How do you know the defendant? We were involved a number of years ago. In a relationship? Yes. Reporter: According to investigators the 37-year-old nurse prwas mcneill's mistress and the key reason that he killed his wife. She is a woman driven, driven in the wrong direction most of the time. I truly believe that she believed that she could control men by an aura about her. Reporter: Prosecutors claim mcneill overmedicated his wife while she was recovering from plastic surgery, surgery he insist she'd have. His goal to kill her and continue his affair with gypsy. Defense attorneys say michele died of natural causes. How did you meet? We met online. Reporter: A year and a half before michele's death. Gypsy testified that over time, their relationship became more heated. Did the relationship become sexual? It did. When was that? In january of 2006. How often were the two of you having sexual relations? We would see each other a couple times a month. Just a month before michele's death she said martin was paying some of her expenses. It was a turning point in their relationship. In march 2007, you move into a duplex? Correct. Closer to the defendant? Yes. Who is paying for this? Martin had a contract on that property. So he is paying for the housing there? Yes. And did he give you a debit card to use for personal expenses? Yes. They communicated frequently. According to the prosecution even exchanged calls on the day of michele's death and 15 texts on the day of h funeral. A funeral gypsy attended. I'm so sorry for your loss. Where she and martin pretend to meet for the first time. My father said, oh, thank you. What's your name, I'm sorry? Oh, she said my name is gillian. Reporter: This was not the first time martin's daughters had heard of gypsy gillian willis. Weeks before her mother's death, alexis said michele had been suspicious martin was having an affair so she asked her to do some late night investigating. While he was sleeping I went online and printed out all of his phone records. And we found a -- this number, that he had been calling a lot. Reporter: A number being dialed day and night. Alexis did an online search. It popped out, the name of the person. Reporter: It was? Gypsy gillian will is. And my mom is like who is this? We thought it was maybe some stripper or something. Who is named gypsy. Reporter: In 2011, she told elizabeth vargas, michele confronted martin about the woman named gypsy. What did he say? This was a nurse he worked with. She is actually going to be renting the property that they had leased. My mom said why have you called her at 3:00 in the morning, all these strange hours? Oh, she works a night shift. My mom wasn't happy with his answers. She really didn't believe him. Did she follow up? She, well, she died. Then just a few weeks later mcneill hired gypsy as the new live-in nanny to take care of the youngest of his children. Martin's daughters did not approve. I said, gypsy gillian willis. I know that woman. I know mom was worried you were having an affair with her. And you are not to bring her in the home. Reporter: He hired her anyway. Martin's daughters say gypsy paid little to no attention to the children. Her focus was elsewhere. I expected her to be focused on the children. I expected her to -- to do things related to that. To, to cook, or clean or take care of the children. What did you see her doing? My dad was cooking. She was sitting there -- staring at my dad. It just was obvious that she, she is just goo eyes at my dad. Martin's daughter bes became uncomfortable. She walked into the house like she owned the place. When I questioned my dad, what is going on, oh, she is a guesten guest in our home. Reporter: His daughters say martin made it clear given a choice between them and gypsy he told gypsy. I was told I needed to leave the home. I wasn't nice to gypsy. We were pushed out of the house. I didn't hatch shoes on. Reporter: If gypsy wasn't a nanny who was she? According to her own family she was no angel. I would kid her to be a deceptive, malevolent, calculating person. They say gypsy used to enjoy killing animals and attacked her mother in an argument over a dog. She lunged forward, bit me. You could count every single tooth mark. Reporter: Martin was captivated by gypsy. Traveling to wyoming to meet her family shortly after michele's death. He said I never loved michele. But I love gypsy. Reporter: Martin didn't stop there. In front of her whole family he pro pose to gypsy. He gave this speech how he loved her from the moment he saw her. Knelt and proposed to her. Gypsy cried. Very fairy tale. In a bad way they were perfect. She believes together. Her sister and martin are a dangerous combination. Together I believe they're capable of killing michele. A pack of dogs. One dog alone, may be malicious. Two dogs hunting are lethal. The pair embarked upon a cruel and illegal plot to send martin's 16-year-old adopted daughter back to ukraine, keep her there to steal her identity and give it to gypsy. They went in to court and changed the birthday. With her daughter in ukraine, the investigators say the pair GATHERED FAKE I.D.s OVER UTAH, Using giselle avenue name. They've were caught and did time in federal prison. Another piece of evidence that drove the daughters and investigators to suspect martin in the death of his wife. I don't know exactly what he did, but I do know he killed her. Reporter: Just one month after being released from jail, martin was arrested again. This time for murder. Now, gipypsy is testifying for the prosecution and could be key to martin's fate. Defense attorneys say she was martin's mistress and the affair wasn't a motive for murder. It was a very casual thing. Whenever we had time. And it could be arranged. Okay. And it was -- I think we probably had sex half the time. I mean some times it was just lunch. Reporter: Gypsy is expected on the stand tuesday giving more answers that could ultimately determine her former lover's future. For "nightline," adidi roy, los angeles.
This transcript has been automatically generated and may not be 100% accurate.