Keeper of the Spire, or more simply known as "Keeper" by his apprentice, is the entrusted attendant of the Spire and the unusual ancient scrolls and artifacts containing powerful arcane knowledge.
In Keeper of the Spire, the Keeper is first seen in Calliope's flashback to when she first met him. On the streets, Calliope performed magic tricks to a less than enthusiastic crowd. Seeing this, the Keeper recruited her for her perseverance in performing such tricks despite going so unnoticed by the masses; he could see she was the perfect person to become the next Keeper of the Spire after his demise.
Months later, he assists Princess Rapunzel and Eugene to stop Cassandra from acquiring one the objects within the spire's depths.