Who he was
Ji Dan, the Duke of Zhou (周公), was a son of King Wen and brother of King Wu. When Wu died, he ruled as regent for his young nephew, King Cheng, and held the new dynasty together. Centuries later, Confucius held him up as the model of a wise and loyal ruler.
What tradition says
From the Han dynasty on, many commentators credited him with the line text entries, the texts of the six lines of each hexagram. The reason was practical: some lines seem to mention events after King Wen's time. The fifth line of Darkness, for instance, names Jizi, a Shang prince whose story ends under the Zhou. If the father couldn't have written it, the son must have.
That gave the classic scheme: Fu Xi drew the trigrams, King Wen wrote the judgments, the Duke of Zhou the lines, and Confucius the commentaries.
What historians think
There's no evidence the duke wrote the lines. Many scholars see the Zhouyi as gathering diviners' material from several generations, with the duke's name lending the text a founder's authority.
In a reading
When you read the text of a moving line, you're reading words tradition places in the Duke of Zhou's mouth.