Frank Reynolds is a fictional character on the FX television series It's Always Sunny in Philadelphia. Frank is the legal father of twins Dennis and Dee and the roommate as well as alleged biological father of Charlie. He is played by Danny DeVito.
Frank appears to be intelligent relative to the rest of The Gang, but is quite possibly mentally unstable. In episodes his character frequently becomes paranoid, delusional and psychotic. He has also attempted suicide several times, and has consumed a large quantity and variety of drugs throughout the series. Frank styles himself as a master manipulator and frequently takes the lead in The Gang's schemes. He used to be prim and proper, but after he divorced Barbara, he let himself lose all convictions and live with no moral compass or decency.