Hallett was taken by ambulance to Cedars-Sinai Medical Center in Los Angeles after having problems breathing and died there Sunday, following a five-year battle with the heart condition, his agent Pat Brady said Tuesday.
His father, Dave Hallett, was by his side.
"Andy was the all-American boy from Massachusetts," Brady said. "He was a hoot. He was comfortable wherever he went. Girls loved him. He was a very gregarious, happy young man."
Born and raised in Osterville, Mass., Hallett moved out to Los Angeles when he was 23 and worked as a messenger at a talent agency, said Brady.
He went on to become an assistant to Joss Whedon's wife, Brady said. It was when Whedon saw the lanky Hallett perform karaoke that he had him audition for "Angel," a spinoff of Whedon's hit series "Buffy the Vampire Slayer." Hallett snagged the part of Lorne.
Hallett proved himself a fan favorite on "Angel" as the show tune loving, red horned demon seer who runs the karaoke club Caritas and can read a person's aura when they sing, revealing their problems and futures.
The series, starring David Boreanaz as the brooding vampire Angel, ran for five seasons on the WB network before it was canceled, ending in 2004.
Hallett was diagnosed with his heart condition at the end of the show, said Brady, and had been in and out of hospitals for the past five years. He decided to concentrate on his own music, stepping away from acting.
Hallett, an only child, is survived by his father Dave Hallett and mother Lori Hallett.
Copyright 2009 The Associated Press.