I'd say Undyne. She's a really powerful character, which just really wants to help the people of the underground. Unfortunately she suffers from believing that all humans are evil beings who deserve no mercy. Then again, I'd say all the characters suffer from some sort of mentality, or other character trait. Papyrus seems to be quite oblivious to many of the things that happen around him although that could also be denial. Sans just doesn't care about anything until you kill his brother. Toriel secluded herself from the rest of the underground and believes the best course of action for any humans she finds is to keep them with her, although she fails every time which is apparent. Alphys is a genius who suffers from extreme social anxiety and doesn't believe in herself without some sort of third party to motivate her. Asgore is a king stricken with grief over the loss of 2 children and being left by his wife. He also knows of no other course of action than to just kill any humans who enter the underground, although he severely hates when he has to kill. He's just a big softie in need of the right guidance, and maybe a hug.