Compendio: Frank is established as Dennis and Dee’s father when he first appears in Season 2 of "Always Sunny" — though the audience later finds out that he is not their biological father.

The 'Abbott Elementary' EPs talk jokes cut from 'Always Sunny' crossover, including backstories for Frank and Mr. Johnson.

We always knew that It's Always Sunny in Philadelphia's Frank (Danny DeVito) would easily match Abbott Elementary's Mr. Johnson's (William Stanford Davis) weird. But we just didn't know how much.