Leo Caesius is one of AH.com's more intellectual members; he is known for his immense knowledge of linguistics as well as being fluent or semi-fluent in a vast array of languages, including Neo-Mandaic. He has a particular interest in the Middle East in general, and a gradually degenerating obsession with the Mandaeans.

A popular pastime is 'finding Leo's umlaut', referring to his real life name. He rarely replies to posts referring to this.

He's usually considered fairly left-wing, or perhaps it's more accurate to say that he just distrusts the current Republican leadership in the US. His most common right-wing political foil in discussions is Paul Spring.

Leo was elected President of AH.com in the 2006 elections but chose not to participate in the 2007 elections.

The Series version of Leo Caesius has him being the main computer for the AH.com ship and occasionally plotting the destruction of the crew when they annoy him. Although a robotic body was created for him, Leo tends to remain on the ship, due to the fact that once he downloads into the robotic body the ship cannot function.

Leo took on the Hermione Granger role in Luaky Commer and was 'C-Leo', the equivalent of C-3PO, in AH.com Wars.

Part of the cast in Doctor What‘s Extinction Event, in which he appears as Dr. Charles Leon Caesius.

Leo also inspired the character of 'The Linguist', an antagonist from Thande's There'll Always Be An England - a spur of the moment decision on Thande's part, thanks to a topical joke about Leo supposedly burning his university building down in real life, at the same time as a mysterious plot device fire had happened in that book.