Hebrew Pele Yoetz, the first of the four throne-names in Isaiah 9:6. Pele is not "wonderful" in the weak modern sense ("oh, how nice") — it is the word for divine miracle, the astonishing, the supernatural (same root as God's "wonders" in Exodus). Yoetz is counselor, advisor, the one whose plan is followed. Together: "a Counselor whose counsel is a miracle."
To call Christ Wonderful Counselor is to say His strategy is perfect and His wisdom is divine. Human counselors — even the best — give advice limited by their information, their bias, and their lifespan. Christ counsels from omniscience. "In Him are hidden all the treasures of wisdom and knowledge" (Colossians 2:3). His plan at the cross — use the worst evil in history (deicide) to accomplish the greatest good in history (atonement) — is counsel no finite mind would have conceived. Isaiah later writes, "Who has directed the Spirit of the LORD, or what man shows Him His counsel?" (Isaiah 40:13) — no one; God is the counsel-giver, not the counsel-taker. For the believer, this title is intensely practical: when you don't know what to do, your King is not puzzling over it with you. He already knows. Bring every decision to the Wonderful Counselor.