In my opinion, Mariana is perhaps one of the most criminally underrated characters in the TellTale games, and here's why I think that.
1. Mariana has some of the most memorable scenes in the third season. Sure, she only appeared in one episode, but there were a number of moments involving her that stood out to me. The first one is, of course, her death. Sure, we've seen kids die (Duck comes to mind), but we've never seen a kid be actually murdered on purpose. It's rather shocking to see imo.
The second scene takes place at the beginning of the game, where she goes to get Salvadore water, and Mrs. Garcia tells her that she doesn't have to get water, as Salvadore passed. Mariana actually says that Salvadore is awake, which actually is one of the most chilling moments in the games.
Then there's her determinant burial scene where, (if you don't count Gabe's determinant death), is one of the saddest moments in the game. You're literally burying a child that is family. Imagine that.
2. Mariana is not annoying. The reason I didn't care for Gabe much is because he was whiny and rebellious, which I understand, but I still don't like. Mariana on the other hand is a well-behaved kid and actually likable. She's just so...nice, and she kinda reminds me of a pre-apocalypse Clementine with a few changes.
I know there aren't a lot of that, but it's why I think Mariana is underrated.