A midshipman who did not show up this week for his court-martial at the U.S. Naval Academy and who is still missing is now believed to be accompanied by his 18-year-old girlfriend.

Clayton Matthew Lewis, 20, disappeared from the academy on Monday, the day before he was scheduled to face trial on charges that he had stolen goods worth $8,000 from an academy store.

His girlfriend, Jennifer Lesley Simmons, was visiting him over the weekend and did not return to her home near Dallas as scheduled on Monday. "Mr. Lewis has got to come to his senses and turn himself in," her distraught mother, Diane D. Simmons, said yesterday. "I just don't know what they are thinking about."

Lewis, from Roanoke Rapids, N.C., who has completed two years at the academy, was charged in May with a 16-day unauthorized absence from the academy. Simmons said Lewis had visited Texas during that time and had attended her daughter's high school prom.

After returning to Annapolis, Lewis was ordered not to leave the academy grounds and to report to his superiors every two hours.

Earlier this month, Lewis was charged with larceny, conspiracy to commit larceny and receipt of stolen goods for allegedly taking about $8,000 worth of clothes, record albums and appliances from a store at the academy over a nine-month period. If convicted he could face up to 11 1/2 years at hard labor, forfeit his pay and be dismissed from the Navy.

At 8 p.m. last Monday, the night before his court-martial was scheduled, Lewis failed to report to his superiors. His possessions were still in his room, academy officials said, but Lewis was not to be found. Meanwhile, Simmons said, her husband went to Dallas-Fort Worth Airport to meet their daughter, who was scheduled to return from a visit with Lewis, but she was not aboard the plane.

Simmons said she and her husband did not realize Lewis was in trouble until he disappeared. Lewis had described hardships at the academy, she said, "but I understood that's part of the process they go through there."

A Washington Post reporter who happened to meet Lewis at the academy a month ago said Lewis seemed disillusioned but not bitter about the academy. He said at the time he was eager to leave Annapolis and wanted to go to law school instead of making a career of the Navy.

Lewis said he was unhappy with all the rules a midshipman faces at the academy and that he had been put on report twice for infractions that he characterized as silly. One of these, he said, was for failing to wear rain gear, which was the uniform of the day, because the sun had come out after a brief shower.

Asked then if he would enroll again at the academy if he had it to do over again, Lewis said he probably would because he came from a family that could not afford to send him to a private school.

Simmons said Lewis and her daughter had been sweethearts for four years and described them as "two young kids in love . . . . He has spent summer vacations here and Christmas vacations here. He has been like a member of our family."

Lewis' father, Melvin Lewis, said his son told him in July he planned to resign from the academy this year and go to law school.

Lewis, who last spoke with his son in early July, said he knew his son had been restricted for being absent without leave but was unaware of the theft charges until his son vanished. He said his son had never been involved in theft and his only brushes with the law had been for driving offenses.