For months, fashion designer Martin Gomez tried to warn Selena, the glamorous Mexican American singing star, that her personal assistant was "evil," an obsessed woman who was lying, stealing and sabotaging other people's work. He was afraid, he said, of Yolanda Saldivar.

"The last call I had with Selena, the last call, I told her to be careful," Gomez said today. "It was very weird. I was very afraid of Yolanda. But I never thought she would hurt Selena. I never thought it would come to this."

On Friday, Selena Quintanilla Perez, a Madonna-like performer who seemed poised for full-scale star dom, was shot through the heart at a Days Inn Motel in her hometown of Corpus Christi. Police have charged Saldivar with murder, after she held officers at bay in the motel parking lot for more than nine hours, seated inside a red pickup truck and threatening to kill herself.

It was no secret that Saldivar, 32, a registered nurse from San Antonio, had rearranged her life around the Grammy award-winning singer she so admired. The founder of Selena's first fan club in 1991, she had proved herself so helpful that last year, she was hired as manager of the design house and the boutiques in San Antonio and Corpus Christi that Selena had opened to market the flamboyant costumes that had become her trademark. She had a key to the home Selena shared with her husband and guitar player, Chris; control over the singer's business checking accounts; and, increasingly, bitter words for anyone who tried to question her authority.

On Wednesday night, Selena -- who friends and business associates say had loyally defended Saldivar when anyone criticized her -- was forced to fire her assistant over discrepancies in the company's finances. The next day, Saldivar telephoned Selena and arranged a meeting at the motel for Friday, saying she had some bank statements Selena might need, said Jimmy Gonzalez, director of marketing at Selena's Q Productions studio in Corpus Christi.

"There was no fight when she was released of her duties. She just said, Okay,' " Gonzalez said today in a telephone interview. "Selena, not thinking anything, proceeded to the motel, and that's when the lady pulled the gun on her.

"I think it's a matter," he said, "of when an artist becomes popular, some people become obsessed, especially someone close to the artist. She obviously went too far."

Selena, who was about two weeks shy of her 24th birthday, was wildly popular in the world of Tejano music, a saucy blend of polka, jazz and traditional Mexican accordion-based styles. One of her signature songs was the infectious "Bidi Bidi Bom Bom," and at last year's Grammy awards, she won in the Best Mexican American album category for her "Selena Live!" album. At this year's Tejano Music Awards, she won seven major prizes, including Female Entertainer of the Year, and more widespread fame seemed ensured in her future, Gonzalez said, as she garnered a $5 million multi-album contract with SBK records. She was currently at work on her first English-language album, although she always joked in interviews that, as a Corpus Christi girl, it was her first language anyway.

Her death and its tragic circumstances stunned thousands of fans, who quickly arranged candlelight vigils Friday night in San Antonio, Dallas and other cities where her vivacious music and status as a Latin-American artist who had made it endeared her to many. She was the sort of performer who appealed to both young men and young women, and concert stages were often littered with cowboy hats tossed in tribute by her fans.

To people who knew her more closely, "she was an absolutely beautiful person, not just physically but mentally," Gonzalez said. "She had a big heart for everyone, and that's what cost her her life. She didn't think anyone would be so cruel."

But Martin Gomez, who shared an office with Saldivar for eight months, had quickly begun to have suspicions that Saldivar's devotion was bizarre. Six weeks ago, he left his position as Selena's fashion designer -- their label was "Martin Gomez Exclusively for Selena" -- solely because he could not stand working alongside Saldivar any longer, he said today.

"She was very vindictive. She was very possessive of Selena," said Gomez, 30, of San Antonio, in a telephone interview. "She'd get, like, very angry if you crossed her. She would play so many mind games, say people had said things they hadn't said. So many things would happen to the clothing I was working on. I knew that I had finished a certain piece, but I would come back from a trip to New York and the hems would be ripped out. It was very strange."

Gomez described Saldivar as "a little bitty thing, very dowdy. She kind of looked like a man, she looked like a munchkin." He noticed, he said, that she had a fondness for rental cars and cellular phones, and that, before taking a recent trip to Mexico, "she bought a ton of clothes," feeding his suspicions that her accounting practices were questionable. On one occasion, he said, after some fashion models had not been paid for their work, she told Abraham Quintanilla, Selena's father and manager, that she had passed on the check he had given her for payment to Gomez, a claim he says is not true.

She seemed to have very little personal life, he said, and her face would light up happily when Selena would breeze into the office, laughing and joking, on top of the world about her rocketing career.

"Selena treated her very professionally," Gomez said. "Selena was a darling girl, very sweet, very sweet, but I never, ever thought Selena treated her special. She was nice to all of us. But it got to the point where Yolanda was the leeway between us and Selena, she was the voice, and she tried to shut everyone out."

Gomez said he was often struck by Saldivar's unpleasant attitude. One time, out of the blue, he said, she approached him and asked, "Sincerely, do you think you're better than me?" Another time, he said, she asked him, "If I had money, would you like me?" She once ordered Gomez's assistant, he said, to clean out Selena's file cabinets and proudly took credit for the work when Selena expressed her gratitude.

"Selena was busy and she was naive; she was a 23-year-old kid," Gomez said. "There's so much, in retrospect. I was just yelling yesterday to a friend, I specifically told them this lady was evil and nobody ever believed me.' They'd say, You just don't know her.' "

He thinks Saldivar, while outwardly adoring toward Selena, was secretly jealous of the beautiful singer. "Deep down inside," he said, "she knew she was nothing. That's horrible for me to say, but that's what I saw." CAPTION: Tejano star Selena, left above, watches as personal assistant Yolanda Saldivar speaks to crowd last year in San Antonio. At right, young fans of the singer pray outside her home in Corpus Christi. Saldivar was charged with murder after holding police at bay for nine hours following shooting.