I'm going to emphasize things other reviews have not, if you're still on the fence. YOU are the character. No, I don't mean the avatar on the screen. I mean YOU. As such, all of your choices matter not only in the gameworld, but to yourself. In that, Undertale is probably one of the most immersive RPG you'll ever play. Oh, and its charming, original, has 3 very different branching endings with minor changes based on decisions, engaging combat, atmospheric music, and tons of Easter Eggs if you look. Get it.
