Sean is the second human to fall down into the underground. He did so to find his missing father, he was told of his father as if his father was just a fairy tale.
He enjoys drawing and hanging out with Sans. He maybe only 7 but he is smarter than other children his age.
He is unaware of the fact that he is half monster, it is unknown if he has monster magic skill.
He is a bit fearful and shy, but once he knows you he opens up and expresses fondness of cats. He will stop by Temmie Village just to pet a cat.
He enjoys puns and drawing, he finds Sans funny as heck. He is near sighted and wears glasses. He get headaches from not having glasses. His glasses broke in the fall and decides to keep his eyes closed to keep himself from having headaches.
He really likes Toriel and is very fearful of Asgore, he just wants his dad. When times call for it he can be brave, and other times he just wishes to be safe in his father's arms.
When he first met Gaster, He was unaware that Gaster is in fact his father. But once he learned it, he just keeps close to him and hugs him. He really comes off as needy to someone whom isn't in the known that Gaster is his dad.
Sans is like an Uncle to Sean. When they first met Sean was very lonely and lost and Sans decided to help the poor kid out, in doing so he gained Sean's trust. The two of them pun it up and draw together.
When he first met him, he was fearful of him and the glitches that Gaster's memories coming through didn't really help but Sean sees him as his dad even in this form without his Gaster memories.
Sean himself can't make a genocide run, Gaster can however, but it is only due to if Sans is killed right in front of him by another monster. Chara doesn't seem to care either way and still shows themselves to Sean.