Before the Old Kingdom, which ruler unified Upper and Lower Egypt?

King Narmer (or traditionally, King Menes)

Explanation:

Ancient Egyptian tradition credited Menes, now believed to be the same as Narmer, as the king who united Upper and Lower Egypt.

Sometime around 2686 BCE, Upper Egypt came north and invaded Lower Egypt, unifying the two kingdoms under a single ruler who took the title of pharaoh and wore a double crown. Most accounts attribute this moment to King Menes or King Narmer.