I know a small amount about Marius and the reforms he made. However there seem to be few records of his personality. Could anyone direct me to somewhere with such records? Or does anyone have any knowledge of him themselves?
Plutarch wrote one of his Lives about him. So that could fit your bill with the requisite boulder of salt for Plutarch on personality.
As /u/maxotmtns has recommended, Plutarch's Lives is the best primary source for personality. However, bear in mind that everything he wrote was filtered through a) historical bias (including Sulla and Caesar's agendas) and b) his desire to bring out moral lessons in the personalities. Try the classics archive or Project Gutenberg for translations.
Appian's The Civil Wars and Plutarch's Fall of the Roman Republic also have small sections on Marius.
I'd personally recommend Colleen McCullough's The First Man in Rome and The Grass Crown for a good, if fictional, representation of his character and actions.