Kevin (Probably) Saves The World set forth the mystery of whether the title character's angelic friend Yvette is real or not. Kevin (Probably) Saves The World is a comedy-drama starring Jason Ritter (Freddy Vs Jason) about a suicidal man who moves in with his estranged sister Amy and niece. After he touches a meteor he's able to see a being called Yvette, who claims he's been tasked with locating other righteous souls and saving the world.
Kevin (Probably) Saves The World was originally titled The Gospel According To Kevin, though this was later changed to emphasize it wasn't strictly a religious series. The show was also a good showcase for Jason Ritter as a performer, allowing him to play both the comedy and drama of the concept. While the actor has done solid work in shows like Gravity Falls and The Event, he hasn't quite found a breakout role yet.
Sadly, Kevin (Probably) Saves The World didn't prove to be that role either, with the series being canceled after one season - on a cliffhanger, no less. While the concept had shades of Touched By Angel or Joan Of Arcadia - another show that featured Ritter - Kevin (Probably) Saves The World was more of a breezy comedy and attracted good reviews. A big part of this was down to the central relationship between Kevin and Yvette, his invisible protector. Yvette was originally played by comedian Cristela Alonzo (Cars 3) before being replaced by Kimberly Hébert Gregory (Vice Principals).
A major part of the conceit of Kevin (Probably) Saves The World is the question of whether Yvette is real or just in Kevin's head. When the show begins nobody else can see her, which often makes Kevin look crazy for talking to nobody. Yvette convinces him to do good deeds and seek out the other righteous souls like him, but even she has no idea where they are. She also starts to gradually develop more human traits the more time she spends with Kevin.
In the Kevin (Probably) Saves The World season finale - which turned out to be the final episode - Kevin's sister and niece look at security footage of Kevin seemingly talking to a ghostly shape. Kevin managed to find some other righteous souls towards the end of the season and later confesses his crazy mission to Amy (JoAnna Garcia, Fist Fight). Naturally, she doesn't believe a word of it - until Yvette appears in front of her too, stating she needs Amy to help Kevin on his quest. If there was any lingering doubt about Yvette being real up to this point, the fact Amy can see her too should remove it. It's a pity the show ended on this beat, however, since it opened up a lot of possibilities for Kevin (Probably) Saves The World season 2.