Recs.
Updated
SpecsUpdate
Pros
Pro Amazing characters
The characters in the game are written well, each with their own unique personalities. Some may be too shy, and will lower their hit points when you attack them in battle, just to avoid offending you. Others are deeply caring and do their best to help and protect their friends and family, including you as the player. Many of the characters are quite funny in their dialog and the unexpected choices they make throughout the story, including the ways they react to your choices. Almost every character is incredibly memorable and enjoyable to get to know better.
Pro Your choices lead you down different story paths
Depending on your choices, your playthrough can change in drastic ways. If you don't want to kill enemies in battle, you can choose to have a dialog with them instead. Doing this consistently is basically a pacifist playthrough where you don't kill anyone, and characters respond to you in certain ways based on that. Or you can go for a genocide playthrough, which is basically what it sounds like, where you kill everyone you come across whenever you get the chance. This can have serious consequences, since the game does its best to make you feel bad about this approach, by having certain characters hate you. It's an interesting way to get to see the world and story from different angles.
Pro Many interesting secrets to find
There are many secrets to discover in the game, like hidden mechanics that might take you by surprise. It may take a couple of playthroughs to find them all, but once you do, it may completely change how you see the story, the characters, and a lot more. There's a lot more to Undertale beneath its simple-looking surface.
Pro Incredible soundtrack
Just like the simple but pleasant graphics in the game, the music matches this with a retro-sounding score that fits right in with the art style. There are a lot of catchy tunes that only use a few sounds to make up memorable songs. The simplicity really adds to the charm, with some of the songs easily able to get stuck in your head for days at a time. It's nostalgic enough for older gamers who grew up in the early eras of gaming, while still having enough to pull in newer gamers who aren't as familiar with games from the 1980s and 90s. There's something here for everyone.
Pro A heartfelt story about giving kindness a chance even when at first it might seem that the recipient doesn't deserve it
At the heart of Undertale is the theme of everyone being generally good on the inside, despite common misunderstandings and outward appearances. In this particular tale, you (a human boy) are thrown into a land of so-called "vicious" monsters and given a very special morality choice. You can kill the monsters and gain experience in traditional RPG fashion, or you can avoid killing monsters and resolve conflicts by talking things out peacefully.
Rather than each monster being a generic NPC as in traditional RPGs, each monster in Undertale is a unique individual with their own personality and story to tell. By choosing not to fight, you can instead get to know these monsters for who they really are, understand their motives, and learn how each one grew to be despised by humans. Each one contributes to the story in their own way, and by sparing their life, you will grow deeper and more meaningful relationships with each inhabitant of this strange world.
Cons
Con Replaying can be a chore
The first time going through the game it's a nice experience, especially when you know nothing about the characters, story, mechanics, etc.. By your next playthrough, you already know all the jokes and plot points. Even when taking a new route, it can take a while for you to notice anything different from one playthrough to the next, making things feel like a drag.
Con The visuals could be better
Much of the game is plain-looking with simple graphics, but sometimes the visuals are just awful. The pixel art can be charming for some players, though it's hard to ignore when certain areas are barren or have ugly textures. It's notable that a single person created the whole game; it would have been nice if they'd had more time or resources to clean up some of the visual problems.