Marc Bolan, Bowie's first friend and muse on his journey.
There are striking similarities in their early careers--both spent most of the 60s as struggling, aspiring pop stars, not breaking through and finding fame and recognition until the 70s began. Both were born in the same year, and both adopted striking stage names (each would give their sons first names that rhymed with their famous stage names: Zowie Bowie and Rolan Bolan). Both were instrumental in popularizing "glam rock", neither having found much success trying their hands at folk rock and "hippie" music.
They would maintain both a close friendship and rivalry throughout their parallel careers. Bolan double as rival and muse, both keeping Bowie on his toes and inspiring the character Ziggy Stardust ("Lady Stardust" was originally called "Song for Marc"). Both experienced great success circa 1972-73, but Bolan's star began to decline along with glam rock, already having become a parody of itself and well on its way out by 1974, giving way to disco and punk rock by 1976. Bowie was able to adapt and re-mold himself, remaining on the vanguard when he defected to Berlin to record Low and Heroes in 1977. Bolan by this point was struggling to remain relevant, hsving become host of his own variety show. Bowie guested in 1977 and performed his new single "Heroes", also performing a jam with Marc's band during the closing credits. Bolan was beginning to be recognized in the growing punk movement, so it's possible he would have experienced a rebirth, casting himself as a sort of punk forefather similar to Iggy Pop and Lou Reed around the same time. But fate intervened and we'll never see how his career might have evolved and paralled Bowie's into the 80s. Bolan was killed in a serious car accident about a week after he filmed his variety show with Bowie.
Bolan's girlfriend and son were suddenly sent into poverty--Bolan was still legally married to his first wife, and so his new son and his lover were entitled to zero if his estate. He had been in the process of setting up a trust for his son and getting his set up to receive his inheritance--but the car crash came before he had finalized any of this.
Bolan's girlfriend and his son moved to LA and struggled to make ends' meet. She had numerous health issues from injuries suffered during the fateful car crash. Bowie would call them regularly, always asking what he could do. He sent money, never asking for repayment and eventually paid for Rolan's education. This was a final gesture to his former friend and competitor. They may have had a strange relationship, appearing as arch rivals in the press, but ultimately their relationship had been driven by a deep friendship, likely based on sharing so many of the same experiences in their early recording careers.
Marc: "Your shoes are crap"
Bowie: "Well you're short"
If I were going to film a Bowie Biopic, I'd likely focus on 1964 to 1977 and their relationship. Ending it around the point Bolan died. There would be no Bowie as we know him without Bolan.