V4P5
While Leonard was in the capital, his little brother Arven Stewart, was filling in as the head of house.
There should be a comma right before "Arven Stewart", or no comma after it.
While Leonard was in the capital, his little brother, Arven Stewart, was filling in as the head of house.
(The principle description is "little brother", and "Arven Stewart" is a parenthetical clarification of this.)
While Leonard was in the capital, his little brother Arven Stewart was filling in as the head of house.
(The principle description is "Arven Stewart", and "little brother" is additional description of him.)