From 1968 until his departure to appear in Satyricon, he was a member of "the tribe" in the original line-up stage production of Hair, directed by Tom O'Horgan, written by Gerome Ragni and James Rado, music by Galt MacDermot.
Keller also worked as a runway and print model for 14 years and acted in the New York underground scene with Andy Warhol.
Fellini's long-time employee and screenwriter Bernardino Zapponi wrote of Keller's representation: "[He] has a very photogenic smile and is full of knowledge as required by his role.
and had the male lead role opposite Jane Birkin in the Antonio Margheriti-directed horror movie Seven Deaths in the Cat's Eye; in 1975, he had the main role in the science fiction thriller Lifespan, with Tina Aumont and Klaus Kinski, followed by Catherine Breillat's A Real Young Girl (1976).
He appeared in 1974 in two productions for Italian television, Miklós Jancsó's historical drama Roma rivoule Cesare and in the series Orlando furioso, based on Ludovico Ariosto's epic poem.