Billy Corgan is reflecting on his friendship with the late Lisa Marie Presley, and he admits there's one regret he has over his working relationship with her, or lack thereof.During an appearance with The Smashing Pumpkins on SiriusXM's, the rock band's frontman opened up about the tragic loss of Elvis and Priscilla Presley's only child, after she died in January at the age of 54.Following her death, Corgan was among those who performed at her funeral, held at Graceland in Memphis, Tennessee, where he performed the band's 1998 single, «To Sheila.»«She was awesome — that was a tough one,» the 56-year-old musician said. «That was really hard for me because she was a one-of-a-kind person and the world really didn't know her because they, of course, always knew her as Elvis' daughter, but she was a really, really cool, unique person.»Corgan and Lisa Marie briefly worked together.