A "staunch Green Mouser", Isaac was a fundamentalist who ranted and raved about anything and anyone that he felt mocked the Green. Always unhappy, he rarely smiled. He held his son to strict standards and beat him often, believing the discipline was good for him.
He resented Jenkin for being the indirect cause of his wife Meg's death, for she died giving birth to their son. When he learned that Mahooot carried Jenkin away and devoured him, however, he was devastated.