Hey remember that time Greg Serrano became a whole entire different dude? OF COURSE YOU DO. WE ALL DO.
Crazy Ex-Girlfriend blurs the lines of fantasy, reality, and also ghosts of alive people tremendously well. But its biggest leap into the multiverse came courtesy of one major recast. Italian bartender and disaster human Greg Serrano (Santino Fontana) was Rebecca's (Rachel Bloom) on-again-off-again love interest and utter "sh*tshow" for the first two seasons until he got sober and left West Covina to move to Atlanta and attend business school. But in Season 4 he returns. At least, Greg returns. Fontana does not. Instead, Greg is now played by Skylar Astin.
The show takes great pains to tell us this is a matter of Rebecca's perception and growth, that she literally sees Greg as an entirely different person — meaning both she herself is a different person and Greg is literally, I cannot stress this enough, a different whole entire g*ddamn human being. No one else sees this change save for Rebecca and the audience.
And that is because we have been given Greg Serrano of Earth-2, an AU Greg with his life together, a little less snark and a lot less darkness, but the same song in his heart (well Rebecca's imaginary version of the song in his heart), and the same commitment to that T-shirt under an open button-down shirt life.
Is he an imposter sent from this other universe to steal our hearts? Is his motivation even more insidious? Did he kill Santino-Greg, perhaps as vengeance for his cruelty to Princess Anna of Arendelle? Am I positing that Crazy Ex-Girlfriend and Frozen exist in the same universe? I'm just saying we don't know how deep this goes. Stay alert. If handsome singing dudes show up, you're just going to have to really investigate. DIBS ON INVESTIGATING THE HANDSOME SONG BOYS.
...You know. For our Earth's protection.