Leonard Davia, also known as the Jester of the Prime Moon, is Isaac's friend and teacher. He is also the Sage of Darkness. He is a young man with long purple hair and black robes.
Isaac and Trista meet Leonard early on, where he teaches Issac the first Dark Rune. Later, Leonard teaches Issac how to combine Dark Runes with other elements for better combinations. He later teaches Isaac the second Dark Rune. At the Balance Temple, it is revealed that Leonard is the Sage of Darkness. He and Russell hold off the Diva of the Twilight long enough for Isaac and Trista to escape. He later meets them at the City of Ruins, but as a corrupted Sage, and so is forced to battle Isaac. When Isaac wins, Leonard teaches him the Lost Magic of Darkness. After discovering that Twisted Wand Castle is barred off, Leonard takes Isaac to meet Leonard's "mother", an elder Dragon. His "mother" teaches Isaac Trio Runes. After Isaac beats the Diva, Leonard leaves for the monster realm to be with the world's monsters.