Timothy G. Byrne was a classic, jet-setting investment banker, driving a black BMW that smelled like stogies the day after he bought it. "I used to joke with him that he just didn't want to spend any time alone in his apartment," said his brother, Sean Byrne, one of his nine siblings. A week ago Friday, after finishing up work selling bonds at Sandler O'Neill, Timothy Byrne, 36, left the office with a colleague in his BMW "loaded with golf clubs and eight cigars," Sean said. He played nine holes of golf on Long Island, came back into the city for a date, spent Saturday in Montauk with another brother and his wife, and then met Sean for a Jets game on Sunday. "That's the way it was with him," Sean said. "He always showed up, no matter how busy he was. Maybe not on time, but he was always there.