Sans’ character development.
Sans is a really well-developed character. We know nothing about him, and yet he makes up so much of the Undertale fandom’s obsession.
Think about it. You’ve just left the Ruins, shutting Toriel out of your life forever, or at least you think so. There’s snow, there’s a creepy forest, an endless creepy droning. It’s unsettling, but you can’t turn back. The path is narrow. The only way you can go is forwards.
You keep walking. Crack. A stick breaks. That stick that you couldn’t even lift. How could that just happen? A little freaked out, you keep moving, hoping to leave behind whatever it was that snapped that stick in half.
Stop. What in fuck’s name was that silhouette? That didn’t look anything like Toriel, and whoever it was had been heading towards you. Someone’s chasing you, but you can’t see them and you can’t go any faster. Oh, wait, what’s that? A bridge! You must be close to safety, or at least civilisation. Wait. Shit. Why aren’t you moving? Something’s wrong. Oh fuck, it’s your stalker. You can’t move. They’re heading towards you, they’re talking to you, oh god, you’re going to die...
And there he is - a whoopee cushion in his hand and a grin on his face, Sans reveals himself to you. He’s silly, he’s fat, he’s got a Patrick-y voice. You feel much less intimidated now. Sans is cool. Sans will lighten the mood. He’ll be your friend.
So you carry on playing, watch the hilarious cutscene with Papyrus and the lamp, all the while thinking what an absolute godsend this comic relief character is.
Hold on a moment... I’ll be up ahead? He just walked back to where you came from and he’s nowhere to be seen. ...Huh, he really was up ahead. This guy knows his way around. This doesn’t seem that suspicious to you. Until you go down to the cliff face where Gyftrot lives. Seeing Sans once seemed pretty normal, but then... how the hell did he get there?
You carry on, go to Snowdin, have a nice time there. Wait, why’s Sans’ door glowing? Better not question it.
Carry on, carry on. Go to Waterfall. Oh, there he is again! He’s selling his illegal hot dogs from a sentry station that has snow on its roof. Well, maybe some snow blew inside. That’s all right.
You go to Grillby’s with Sans. That’s a convenient shortcut. After he leaves, you decide to talk to the royal guard dogs just to milk for extra dialogue.
“He loves to appear without moving.” Wait. That’s definitely not normal. Monsters can’t teleport... can they?
You keep playing, and Sans doesn’t appear for a while (save for when he sells you hot dogs at his stand in Hotland... which also has snow on it) until he takes you through another shortcut to the fancy MTT restaurant. He talks to you, tells you about his promise. You nearly shit your pants. But he’s joking, right? Of course, he’s always joking. But something’s different. All these encounters, all the hints - something about this guy doesn’t seem normal. Something about him is creepy, it’s mysterious. You’re not sure you want to find out what.
...But you mine for information anyway, because you love this sort of thing. Get to the judgement corridor at the end of the game, hear Sans’ speech twice, get the key to his room. Let’s go back to the bros’ house. You go, and... What the fuck! It’s dark, you can’t see, and - what the hell did Papyrus just say? He’s pranking you across time and space and his brother acts like it’s nothing. Then you go to his workshop. Strange symbols, blueprints, a machine - all relics from his past. Why could they be so important to him? What significance do they have in the game, and why did Sans only let you look when he figured out you’re a time traveller?
Suddenly the memories come back. The things he said. On the pacifist run, he was talking about how you haven’t died ‘a single time’ At the end of the neutral run, he tells you he’ll see you later. If you hacked the game in a certain way, he’ll know. That time when you made it to the end of a genocide run, he started talking about an anomaly in time and space... Timelines... It’s all becoming clear. He knows things that nobody else does. He knows about SAVING. He knows about RESETs. He knows about the timelines.