In Batman #16 (February 1943), a third character is introduced to the “Batman Family”: Alfred the Butler. He was created by Don Cameron. Alfred starts out as an overweight aspiring criminologist who stumbles onto Bruce Wayne’s secret identity. Batman and Robin accept Alfred into their lives.
Alfred is the beginning of what I will call the “superhero support team”. That is, allies of costumed adventurers who don’t go out costumed adventuring, are not exactly sidekicks, and are not romantic interests either. Instead, they help the costumed adventurers achieve their goals through other means. Hm, maybe Etta Candy is the first such character? On TV, the “support team” concept was cracked by Smallville (ok, who am I kidding, Buffy the Vampire Slayer) and really came into its own in the Arrowverse (Arrow, The Flash).