He has a short and childlike body, he is under 10 years old and has well kept shoulder-length indigo hair. Like his ancestors, he has heterochromatic blue and yellow eyes.
He speaks quite politely. Though he was usually cool and collected, there were moments where his childishness would show such as when he stamped his feet in frustration.
Karl was born out of wedlock as the son of Roswaal J Mathers. During the Demi-Human war, he remained at the Roswaal Mansion, and was raised by Clind. With Julia kept occupied by her role as the Royal Court Mage during the attacks on the Astrea Mansion, Clind continued to raise Karl. Following the unexpected death of his mother, Karl was possessed by the original Roswaal, and inherited the Roswaal estate, along with its assets.
- He created a theory that allows objects to be imbued with yang magic buffs.
- Starting with his original body, Roswaal has been changing his middle initial every time he possesses a new descendant. His original initial was A, making his current body his 11th one.
- All Roswaals middle names follow the English alphabet thematically.