Frisk, the main character, is your local pessimistic and angsty teen. The best idea I can give is Nicole from "Class of '09".

Chara is an endlessly cheery, positive, and light-hearted soul, although they are prone to moments of "out-of-character" anger and depression.

Flowey is largely like Chara, but more starry-eyed and curious about the world. He also has a flower bud growing with him, whom he has named Buddy.

Toriel is a mysterious figure, with the monsters of the Ruins being weary around her due to her elusive nature.

Napstablook is still a shy ghost, although they like writing stories and music for their cousin. Sometimes they can be found hosting a DJ party, even if it's scary for them at times.

Sans is your local prankster, enjoying minor pranks and childish games. He is never found without his yo-yo.

Papyrus is the strongest, most determined skeleton you will ever see... Or so people think. He actually doesn't want to capture humans, although, after a misunderstanding and a few lies here and there, he is known as 'the human hunter.'

Undyne is not the head of the Royal Guard... But instead, the head of the Royal Scientists. However, her passion has caused some... accidents. Which she has to make up for.

Temmie is fine! Totally not overworked and depressed! Nope! Just silly!!! :3

Alphys is the head of the royal guard, but she doesn't act like it. Still, the shy and anxious lizard we all know, monsters have doubted her strength and courage. After all, she seems... Preoccupied.

Mettaton is the Underground's star... Although, in reality, most of his power comes from his cousin's work and support. Without them... He is nothing.

Asgore is the king. He rules. Others follow. That is that.