Over the years, there have been lots of nominees for the title of "Fifth Beatle." This article lists the names most commonly mentioned. Was it producer George Martin? Manager Brian Epstein? Former bassist Stuart Sutcliffe? Studio contributors Nicky Hopkins, Eric Clapton, or Billy Preston? Business manager Neil Aspinall? Press officer Derek Taylor?
Wait a minute. What about conceptual artist, singer, and songwriter Yoko Ono? Somehow her name always slips out of this discussion.
That last nominee may draw smirks from those addicted to an ancient and inadequate narrative about whose "fault" it was that the band disintegrated, but consider. Ono is the only one who can claim to have collaborated as a peer on the composition of a Beatle album track: REVOLUTION 9. Ono also supplies lead vocals -- briefly -- on another track, HAPPINESS IS A WARM GUN. Unless I am very much mistaken, she's the only woman to have done so on an official release. (I'm not counting Phil Spector's heavenly choirs on the LET IT BE album, since superseded.) Come to think of it, I can't think of another Beatle track where a non-Beatle sings a line of lead vocal.
Ono is also mentioned by name on COME TOGETHER. Even if she hadn't been, she was obviously and prominently Lennon's soulmate, muse, and artistic partner.
Truth be told, there was no Fifth Beatle. Ono appears to me to have engaged in more actual collaboration than any of the men on the list, though.
If you are still among of those skeptical of Ono's abilities, consider listening to the track YES, I'M YOUR ANGEL.