Millicas is shown to be a well-mannered boy. Despite his young age, Millicas is shown to be very responsible even planning about his future peerage. He also prefers being referred as "Millicas" rather than "Millicas-sama" by the people close to him.

Spoiler

Millicas Gremory is next-in-line to the Gremory Clan after his aunt. He is the son of Sirzechs Lucifer and Grayfia Lucifuge, the nephew of Rias Gremory, and the grandson of Zeoticus and Venelana Gremory. Although his father was Lucifer of the Four Great Satans, his title does not apply to Millicas, as it was only required for Sirzechs and is not an inherent position, thus Millicas' family name is Gremory.