Apart from the pronounced differences in the two men's personal styles, the Hagler-Leonard match has a large element of mystery going for it. Can a fighter as formidable as Hagler be overcome by a man who has been away from his sport as long as Leonard has? Leonard's leap back into boxing - no tuneup bout for him - seems, at first blush, so improbable as to be doomed to failure. Yet, such is Sugar Ray Leonard's mystique that his challenge of a champion as powerful and resourceful as Hagler has become credible.

When the fight was announced, the bookmakers made Hagler the 4-to-1 favorite, but the odds have since dropped to 5 to 2. What has probably tilted the odds is the often-contradictory pronouncements about the fighters' ring styles by laymen and boxing people alike.

Leonard's tactics are expected to be aimed at frustrating Hagler in the ring. ''That's the key,'' says Leonard. ''Hagler is very tight, very sensitive, very rigid. That's his mentality.'' Hagler is not as nimble on his feet as Leonard, but he is a skilled boxer. Through most of his title defenses, he would box his opponents at long range and then, when the toll of punches had reduced them appreciably, move in for the kill. Against Duran in November 1983, though, he was criticized for being too conservative, especially since the fight went the 15-round distance. He took the criticism to heart. In the April 1985 fight against Thomas Hearns, Hagler launched a savage attack right from the outset. For that bout, he wore a cap with the word ''WAR'' on it; for the April 6 match, there is a ''WAR II'' cap. The feeling from Hagler's camp is that in the forthcoming match push will come to shove very quickly if Hagler has his way.

IN A SPORT IN WHICH champions tend to acquire entourages and to lead lives so complicated that they become harmful to their careers, Hagler is an exception.

His prescribed regimen is to train hard, fight hard and then retreat to a zealously guarded private life in Hanover, Mass. There, the fighter and his wife, Bertha, and their five children live in a 10-room house with an indoor swimming pool, a sauna and skylights.

The simplicity of his life extends to boxing as well. In 1970, as a 16-year-old amateur, Hagler walked into a gym in Brockton, Mass., run by two brothers, Pat and Guerino (Goody) Petronelli, to become a fighter. He is still with the Petronellis. Pat and Goody co-manage Hagler, and Goody continues to train him. (The Petronellis were friends of Rocky Marciano, who lived in Brockton, and they have managed and trained other fighters, including Hagler's half brother, Robbie Sims, a ranked middleweight.) When Hagler began with them, the Petronellis were small-town guys who ran a construction company by day and a boxing gym by night. During the day, Hagler worked for the Petronellis' company and was paid $3 an hour. ''He needed money very bad,'' recalls Pat Petronelli. ''So he'd only eat what he had to. He'd borrow 50 cents for a submarine sandwich, 25 cents for a soda - we'd deduct it on Fridays. He didn't spend unless he thought it was necessary.''

At 18, Hagler capped his amateur career by winning the national Amateur Athletic Union tournament in the 165-pound class. He turned professional a week later. Once Hagler showed the promise of a world-class contender, the Petronellis were hard-pressed to maneuver him into the big time. There were occasions back then when Hagler worried that his career might wither on the vine. One night in the mid-1970's, concerned about the modest purses he was making and the reluctance of the contenders to fight him, he found himself knocking at Pat's door at 10 o'clock to tell him, ''I don't want to die in no man's gym.''