Though a controversial topic, most scholars agree that was likely the "fifth beatle," as he create many of the overdubs on sgt. pepper as well as arranging and playing keyboards.

Though a controversial topic, most scholars agree that George Martin was likely the "Fifth Beatle," as he helped create many of the overdubs on Sgt. Pepper as well as arranging and playing keyboards.
This is controversial because according to the Beatles, anyone who isn't one of the main four members (Lennon, McCartney, Harrison, and Starr) is dubbed a "fifth Beatle," however, most of them agree that the most important one of them was Martin.