Undyne is the current ruler of the Underground, and a former captain of the Royal guard.
She owns a crown that is made of ribs, and she is always wearing it. Instead of eye-patch, she wears a bandage, sometimes portrayed as smeared with blood, but in the most recent pages it's clean and white. Her armor is purple, pointy, and has a golden delta rune on the chest.
Her personality isn't much different from her UT counterpart, but she's more serious, and more willing to be violent. It's also implied that she may be insane, like many other HT characters.
Sans-It was confirmed, that she was the one who cracked his skull, because he wasn't following her orders. He seems to hate and disrespect her.
Papyrus-Their relationship is described as rather physically abusive.