Incredibly compelling character development, with Dr. Gaster in particular being one of the most interesting characters I've ever come across in fiction. There's a famous lesson by Aaron Ehaz, creator of Avatar, that says characters should be "Dimensional, Fascinating, Compelling, and Believable" before any attempts to make them likeable, relateable, or sympathetic. Gaster is the perfect example of how well this approach to creating characters works. His constant attempts at trying to distance himself from his conscience and failing miserably, to the point where he subconsciously sees his state of constant overwork and isolation as a self-inflicted punishment, fulfills all four qualities in one swift, elegant gesture, and the results speak for themselves. It is because he has all of these qualities that Gaster is such an incredibly likeable, relatable, and sympathetic character - not the other way around. It's not just him either - Papyrus's borderline insane stubbornness in his belief that Gaster is capable of altruism, and Sans's attempts to achieve some sort of control over the situation through hiding secrets from Gaster, even at great risk to himself and with no personal benefit save the satisfaction of having outwitted Gaster in some small way, were also incredibly well-executed.

Unfortunately, once Sans and Papyrus escape the lab the story takes a turn for the worse, because Dr. Gaster gets erased from reality, robbing his character arc of a satisfying conclusion right at its emotional peak, and making the subsequent arcs much less interesting, since the previous arc technically never happened even though it did. I think this is especially important since Gaster is framed as the main character, with most of the narration taking place from his point of view. And no, his return later in the comic does not make up for it, because nobody except Sans and Papyrus remember who he is, and due to his experiences in the void his life goals have drastically shifted. He's almost a completely different character at that point.

I wouldn't call the rest of the comic after Gaster falls into the core bad by any means, though. Sans and Papyrus have superb character arcs as they slowly realize more and more about the true nature of the world and try to come to terms with their agitated emotions. But, they're just not as compelling as Dr. Gaster. Now, they WERE almost as compelling as him in the beginning of the comic, but their character arcs are completely interrupted because for most of the comic they don't remember anything that happened, so a lot of their character development from their time as his prisoners was neutered.

It's a lot like how I felt about A Better Place, where all the characters were compelling, but Hannah was so much MORE compelling that she completely overshadowed the rest of the cast. Except at least Hannah was around for the entire comic and had tons of supplementary material focusing on her point of view to give her a bit more presence even when she was just sitting in her tower while other characters got the spotlight. Gaster, Sans, and Papyrus have their story arcs ripped away from them 1/3 of the way through the comic and have to rebuild them almost from scratch, and they're worse off for it.

One last nitpick I have is that I think the monster they accidently summon in the lab is incredibly lame. The design is good but it acts like... some kind of spoiled youth or schoolyard bully. I think it would be way way way better if it was actually treated like the unknowable lovecraftian horror it was clearly intended to be. It gets a little better at the end where it's implied to be some kind of manifestation of Gaster's psyche, but I still found it to be kinda lacking.