It is a diminutive and somewhat derogatory form for character using the suffix -illo.
Character of little notoriety or little importance. Diminutive and derogatory of character. Personajuelo . despicable person .
Diminutive that adds to the character a certain disdain and tinker as in personajuelo, because if it were affectionate we would say little character and if it were a maño the one who speaks would say personajico.