Betty was a fictional construct

It was a voluntary role, but some people got volunteered for it whether they wanted to or not.

Betty was the cell girlfriend. Every night, if you wanted, she would rub your shoulders, suck your dick, tell you how handsome you was, even let you stick your dick in her ass if you was into the nasty.
The only downside was that there ain’t no Betty. Betty was a fictional construct. She was fine in Tyrell’s mind, but she wasn’t real. Betty was more of a title than a person.
And the current Betty was Ernie Collard.
It was a voluntary role, but some people got volunteered for it whether they wanted to or not.

From Tyrell the Ex-Con

Ernie was the cell girlfriend

But he don’t gotta make it easy, and ain’t nobody gonna force him to put on no wig or nuttin’. T’was easier to pay fer cooperation.

Ernie was the cell girlfriend, the “Betty”. That meant he gotta pretend he was a woman named Betty. In theory, he was sposedta put on a wig and talk like ya girlfriend and say thangs like ‘I love you, baby’. In some cell blocks, they Betty would read letters from they real girlfriends in a girly voice.
And jack you off. T’was the main purpose of a Betty.
But Ernie was a drunk and a crackhead when he could find rock, and he don’t wanna be cell girlfriend — he only wanna jack off maybe one or two dingdongs, enough to pay fer a cuppa toilet hooch or a rock. But he ain’t allowed to do that. Betty gotta get off a’ry man who want it.
That man don’t even gotta pay, not technicably. Ernie sposedta do it, the cell paid him jointly. But he don’t gotta make it easy, and ain’t nobody gonna force him to put on no wig or nuttin’. T’was easier to pay fer cooperation.

From The Redneck Ex-Con, Cellmate Memories and Overwhelming Manhood