Prince is nobody's idea of a leading man. He may be a brilliant musician and a charismatic stage performer, but with his short stature, wispy mustache, soft voice and penchant for puffy shirts, he's not exactly movie-star material. Purple Rain ignores this and treats him like a dashing matinee idol, and it only makes his character more unsympathetic. Known only as "the Kid," he's a sullen local Minneapolis musician in competition with a flashy rival played by Morris Day. Virtually the entire cast is made up of Prince's musical associates playing versions of themselves (generally poorly), and somehow Prince comes off worst of all, with the Kid behaving like a petulant egomaniac, treating his band members and girlfriend terribly. The movie ends with his supposed redemption, but there's nothing to indicate that he's changed at all, nor any reason for any of the other characters to suddenly appreciate him.
Not that the plot is particularly coherent before that, either. The big conflict seems to hinge on whether Day will oust the Kid's band The Revolution (played by Prince's actual band The Revolution) from the nightclub where they both play by replacing them with a new girl group led by Apollonia (Apollonia Kotero). But the stakes never seem to be very high, and the Kid doesn't seem to care much about whether the band keeps its gig or not. Throughout the movie, he dismisses a new song written by band members Wendy & Lisa, only to finally embrace it at the end, when it turns out to be "Purple Rain." In real life, Prince, not Wendy & Lisa, wrote "Purple Rain," and Wendy & Lisa left The Revolution out of frustration that their contributions weren't being recognized. That's some heavy irony right there.
So the Kid treats his band members like trash, and when he hooks up with Apollonia, he treats her terribly, too, slapping her across the face when she says she's going to join Day's new girl group. It's implied that this behavior is a product of the Kid's origin in an abusive home (his father, played by Clarence Williams III in one of the movie's only performances by an actual actor, frequently beats his mother), but he never apologizes or appears to change his ways, and yet he gets the girl in the end anyway. When he's not slapping her, he belittles her and takes her for granted, and never seems to support her musical ambitions. Kotero, who recorded a Prince-produced album with girl group Apollonia 6, functions primarily as eye candy, and her love scenes with the Kid are hilariously cheesy.
I've never been a fan of Prince's music; I respect his talent, but his songs don't do anything for me (even when I've seen him perform live). But the live performances are easily the best part of Purple Rain, and given how half-assed the plot is, the movie probably would have been better off as a Prince performance film. That's essentially what it turns into in the last 15 minutes, when it gives up any semblance of story and just throws all the plot points out the window, declaring them resolved despite nothing having changed.