Marcus is a masculine given name of Ancient Roman pre-Christian origin derived either from Etruscan Marce of unknown meaning, or referring to the god Mars. Modern European variants of Marcus include Mark (English), Marcos (Spanish), and Marc (French).

Marius is a Roman family name which was derived either from Mars, the name of the Roman god of War, or else from the Latin root mas, maris meaning "male". Since the start of the Christian era, it has occasionally been used as a masculine form of Maria.

Marcellus is another Roman name, and Marcel is the French form.

In the works of Harry Turtledove, the following character are known only as Marcus:

Marcus (Ruled Britannia), fictional stage character in Shakespeare's play Boudicca in Ruled Britannia.
Marcus (Shock and Awe), fictional Roman soldier in "Shock and Awe".

Characters with the first name Marcus:

Marcus Vipsanius Agrippa, historical Roman General with a posthumous background role in Gunpowder Empire.
Markus Apfelbaum, fictional German rear admiral, minor character in The War That Came Early: Two Fronts.
Marcus Aurelius, historical Roman Emperor appearing in Household Gods.
Marcus Cornelius Cethegus, historical Roman Consul referenced in Household Gods.
Marcus Tullius Cicero, historical Roman Senator referenced in "The Maltese Elephant".
Marcus Annius Libo, historical Roman Consul referenced in Household Gods.
Marcus Puttkamer, fictional German sniper, minor character in The War That Came Early: West and East.
Marcus Radcliffe, fictional backwoodsman in Opening Atlantis: Avalon.
Marcus Aemilius Scaurus, fictional Roman military tribune and POV from the The Videssos Cycle.
Marcus Aurelius Stimpson, fictional tailor and minor character in the The Two Georges.
Marcus Valerius Corvus, historical Roman Consul referenced in Videssos: The Misplaced Legion.
Marcus Wilson, fictional university student in Supervolcano: Eruption.

Characters named Marcos:

Marcos de Niza, historical Spanish clergyman appearing in "Eyewear."

Characters named Marius or its variants:

Marius (Southern Victory), a waiter and minor character in Settling Accounts: Return Engagement.
Marion Morrison, the alternate history version of John Wayne referenced in American Empire: The Victorious Opposition.

Characters named Mark:

Meteorologist Mark, a Denver newscaster and minor character in Supervolcano: Eruption.
Mark Alvarez, fictional criminal, a POV in "Death in Vesunna".
Mark Davenport, fictional American soldier in The Man With the Iron Heart.
Mark Gordian, a pseudonym used by Harry Turtledove in "Hindsight" and "Notes from the General Secretariat."
Mark Stone, a Crosstime Traffic employee and minor character in Gunpowder Empire.
Mark Twain aka Samuel Clemens, historical historical novelist and a POV in How Few Remain.

Characters named Marco:

Marco Plurabello, a banker and minor character in Gunpowder Empire.

Characters named Marko:

Marko Petrovic, fictional Croatian soldier, minor character in Worldwar: Tilting the Balance.

Characters named Marcel:

Marcel (The War That Came Early), a French soldier and minor character in The War That Came Early: Last Orders.
Marcel Duchamp, historical abstract painter appearing in The Great War: American Front.
Marcel Freycinet, fictional plantation owner in The United States of Atlantis.