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. He also often clashes with the Konservative Kalmarite Krew of right-wing Scandinavians on the board.
Leo is one of the most personally kind members of the board, and has been known to inquire after other members, even if they've stopped posting.
He was elected President of AH.com in the 2006 elections but chose not to participate in the 2007 elections. Leo Caesius was also the last executive of AH.com, during the tumultuous day that the Second Republic existed prior to the conclusion of AH.com politics.
After a longer absence, Leo returned to AH.com in 2013. However, at the end of May 2014, he decided to leave the site permanently, and was banned at his own request. The board respected his decision, but many are in agreement that they miss Leo's presence and friendly linguistic expertise.
The AH.com 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.
His backstory was revealed in HEART OF STEEL as being the last remaining survivor of an artificial civilisation that idolised its human creators who had died out; however, as their leaders knew in secret, in fact it had been an AI conspiracy that had killed off the humans. Leaders such as Paul Spring, rejecting all the human history and knowledge Leo prized, made the same mistakes and touched off a war which destroyed their civilisation. Leo survived on the AH.com. His Mandaean obsession is said to be due to being originally programmed on a site in Iraq over a Mandaean tomb which also suffered from a radiation leak.
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.
Appeared in Dial M for Elastic as Doctor Leo Caesius of the North American Academy of Foreign Stuff.
Appears in Doctor What's Snake Oil, as Dr. Loew Kasjusz.