Spurius Carvilius Ruga (fl. 230 BC) was the freedman of Spurius Carvilius Maximus Ruga. He is often credited with inventing the Latin letter G. His invention would have been quickly adopted in the Roman Republic, because the letter C was, at the time, confusingly used both for the /k/ and /g/ sounds. For example, Ruga's own name contained this confusion: SPVRIVS CARVILIVS RVCA (At that time, "U" and "V" were also the same letter). Ruga was also the first man in recorded history to open a private elementary school.
Plutarch is our main source for these inventions, and Quintus Terentius Scaurus confirms the former in De Orthographia. The letter G was already in use before 230 BC; Wilhelm Paul Corssen theorized in Über Aussprache that what Plutarch really meant was that Ruga's elementary school was the first place to assign the C and G to their current phonemes of /k/ and /g/.