[quote=@TheSovereignGrave]
Huh, I never realized how difficult it was to think up a title for a Head of State for a Republic that isn't 'President' or 'Prime Minister'.
[/quote]

"Chancellor", if you're looking for something realistic.

If you want something that sounds more outlandish, than you could probably just make up some official sounding name. The Law-keeper. The Lawful. Primus. Civilian Commander. Centralist. Prime Centralist. Central Federalist. Prime Federalist. Central Republican or Prime Republican.

I like Prime Federalist.