On a Thursday afternoon last July, Chuck Brewer took his girlfriend horseback riding on a Lovettsville, Va., farm and stopped at an open grave he had dug the day before. Standing over the hole, he told Joy Keo the grave would be hers unless she agreed to revive their withering romance.

The eerie scene frightened the 17-year-old high school junior and finally convinced her that her relationship with the handsome and volatile 20-year-old high school dropout would have to end.

When she tried to do just that six weeks later, Chuck Brewer attempted to carry out his threat. He crashed his way into a house where Keo was visiting, kicked in the door to the bedroom where she was crouched in a corner, pointed a revolver at her and yelled, "You're gonna die, bitch!"

Brewer fired four bullets into her body. The first severed her spinal cord and she slumped to the floor. Doctors say she will be a quadriplegic, paralyzed from the neck down, for the rest of her life.

"I was lying on my face and could barely breathe and Brewer said, 'You're dead,' " Keo said from a rehabilitation center where she is undergoing treatment. "I said I'm not going to let him, I'm not going to die for him. That kept me alive then."

Last Wednesday, in a Fairfax County Circuit courtroom packed with friends and relatives from both families, Charles R. Brewer Jr. pleaded guilty to charges of maliciously shooting Joy Keo. He also pleaded guilty to charges of breaking and entering with a deadly weapon, assault and battery and use of a firearm. He could receive sentences ranging from two years to life in prison for the charges. Judge Lewis H. Griffith ordered him to remain in the county jail and set sentencing for March 11.

It was a typical high school romance with an atypical ending. Both students at J.E.B. Stuart High School in eastern Fairfax, she was beautiful and popular, a cheerleader, captain of the soccer team, a pianist; he was tall and quiet, voted "most attractive" boy in the senior class, a first-string basketball player.

But there was another, troubled side to Chuck Brewer that adoring high school girls and team buddies said they never saw. The son of a prominent businessman, Brewer had been seeing psychologists sporadically since he was 2 1/2 years old, court testimony revealed. Doctors said that as a toddler, he would stick his finger in a candle flame and watch it burn, never screaming in pain. He was an explosive child, quick to attack other children.

A fiery temper and a short attention span followed him into adolescence. At 16, he married a girl and divorced her a year later. He joined the Marine Corps and dropped out a month later. He quit school several times. When enrolled in school, he frequently skipped classes, one teacher said.

Members of Brewer's family refused to be interviewed.

Since the shooting, doctors at Central State Hospital in Petersburg have diagnosed his condition as sociopathic, which describes a person with extremely aggressive antisocial behavior. Doctors testifying in his behalf at last week's hearing said he has suffered from brain abnormalities since he was a baby.

But among most of his friends--especially girls--he could be charming. His 1982 high school yearbook declared, "The 'magnetic' Chuck must carry with him at all times a lasso and several vicious dogs to break through the everpresent throng of adoring females surrounding his locker."

That is what most attracted Joy Keo when the two began dating last February. "He was a challenge," said Keo, whose family has shortened their Thai name from Keophumihae (pronounced gow-poom-ha) to Keo. "At school, all the girls would fall all over him. He'd dump them and they'd still be chasing after him . . . I decided that wasn't going to happen to me."

For the first few months they dated, the two got along well, Keo said. They both enjoyed sports--racquetball, tennis, swimming, horseback riding--and "we had good talks," she said.

Keo, who had been a cheerleader for two years, and was usually on the honor roll, dropped cheerleading as a junior, anticipating a strenuous academic year. Then, she said, she began to get tired and bored with a heavy, monotonous course load. Last spring, she dropped out of school over her mother's objections and moved to Florida with Brewer, who also quit school.

Within a few weeks, the schoolbook romance soured. "He got carried away with wanting me to himself and not wanting anybody else around," Keo said. "That led to one thing after another."

Keo began talking about going home. Brewer not only became increasingly possessive. He began telling Keo that he could not live without her, that he would commit suicide if she left him, recalled Marion Keo, Joy's mother.

The turning point came June 29.

"She said she woke up in the middle of the night and he had tied her up with sheets on the bed and gagged her," Marion Keo said. She convinced him to untie her and the two began arguing. According to court testimony, Brewer then beat and punched Keo, breaking her nose. She pretended to accept his apologies, but called her mother after Brewer left for his job at a Lum's restaurant.

Marion Keo, a counselor at the county shelter for battered wives, said the incident confirmed her suspicion that Brewer was dangerous. "A person who hits somebody once is going to do it again," the mother said. "But nobody expected him to go as far as he did."

Keo's mother wired money for a bus ticket home. Minutes before she boarded the bus at a station two blocks from the apartment they shared, Keo called Brewer at work and told him she was leaving.

By car, Brewer followed and tried to coax her off the bus in West Palm Beach. When she refused, he caught an airplane and intercepted the bus again in Orlando, where she again refused to get off. Brewer soon returned to Fairfax and repeatedly called and threatened suicide if she would not see him. The two continued to meet.

"She was the rescuer," said Marion Keo. "He would tell her, 'You are the only person who understands.' He was obsessed with her and couldn't stand being rejected."

Then came the incident at the grave. "It scared me," Keo said. Shortly afterwards, Marion Keo filed charges against Brewer for contributing to the delinquency of a minor. A Fairfax judge ordered him to quit making contact with any members of the Keo family and recommended that he see a psychiatrist weekly.

But he continued to call Joy Keo, and she continued to talk to him and see him occassionally. Her mother said Keo was lured back by his constant apologies and his frequent threats to commit suicide if she left him.

On Saturday Aug. 28, Brewer again tried to call Keo. She was avoiding him, hoping to end their relationship, according to her court statement. That night, Brewer and a friend appeared at the Vienna house where Keo was visiting. Brewer banged on the door, screaming for Keo, according to court testimony. He then left, went to his car and returned, banging on another door of the house.

The friend who accompanied him began shouting to the two girls inside the house, "Let me in. I'm scared. Chuck is acting crazy."

Brewer then bashed in the bottom half of a door and began racing about the house in search of Keo, waving a gun. He chased the other girl out the house, grabbed her by the hair, and demanded to see Keo. He then returned to the house, dashed up the stairs, smashed down the bedroom door and shot her.

Police arrived at the scene minutes later, but because neighbors said they had not heard shots from the house, police assumed Brewer was holding Keo hostage inside. About two hours later, police picked up Brewer several blocks away.

A police investigator then entered the house and heard a moaning sound upstairs. "I thought it was a cat," said police Capt. John R. Eisen. Then he heard a voice whispering, "Help me, help me." Eisen found Keo in the bedroom, lying on her face, her white blouse soaked in blood.

Joy Keo spent seven weeks near death in an intensive care unit. Two weeks ago she was transferred to the Woodrow Wilson Rehabilitation Center in Fishersville, Va., where she is undergoing physical and occupational therapy.

She has resumed some of her high school courses, reading her texts with the help of attendants. She hopes to graduate with the class of '83. Doctors say, however, that she probably never will walk or use her hands again.