Eli is a FitzFaeren.
Eli hid in Grandfather's Room for some time. He presumably used The Cupboards to get to other places. Eli is only active at night. Henry York opens his door while he is not there. When Henrietta goes to close his door, she sees him and tries to chase him through the main cupboard. She is taken to the Lesser Hall of FitzFaeren where she manages to force him to tell her his name. Henry then comes through the portal. In Dandelion fire we find out that Eli is a traitor to his kind and gave three very important relics that had the power to protect the city to Henriett’s Grandfather, causing the fall of the Fitzfaeren. He later swears to Caleb that he will protect Hylfing and dies fighting Darius.
He is short and old, with glasses.