The eldest son of Cecilie. From the beginning, Gwendal has objections to Yuri becoming the Maoh. He is a skilled tactician and is capable of using earth magic and can cast barriers. When not otherwise busy, Gwendal indulges in his hobby of knitting and sewing but often his creatures look nothing like what they're supposed to (for example a bear got mistaken for a black pig or a cat that looks like a raccoon). He is adamnant that knitting helps him sharpen his concentration but actually its a stress reliever for him since childhood. He is also quite fond of cute animals or cute objects such as the dolphin keychain (Bandou-kun) Yuri received at the dolphin show and later gave to him. His childhood best friend is Anissina von Karbelnikoff, an inventor who often tests out her inventions on him, much to his displeasure. Anissina is also the one who taught Gwendal how to knit. Gwendal's left eye is a key to one of the four Forbidden Boxes, the "End of the Earth".
Gwendal is a tough guy except when it comes to his childhood friend, Anissina, and his brothers. He is afraid of Anissina and often hides from her so he can prevent being used in one of her crazy experiments. He is even seen sweating in panic at the thought of her. While he does not outwardly show much affection towards his brothers, he shows in times of great stress that he does care, an example being his decision to stand against the Original King to save his little brother, Wolfram. Jozak, who was sent to locate one of the Four Forbidden Boxes, tells Conrad that Gwendal really cares for Conrad's wellbeing and doesn't want Conrad to worry despite the Original King's wishes for Conrad to find the Boxes. His ancestor, Siegbert Voltaire, fought along the Original King.
When he was younger, Gwendal despised humans, a habit called the Mazoku Pride by Dunheely Weller. On Dunheely's last journey to a nameless village, he told Gwendal to enjoy the ride before their last duel. Upon arriving in the village, Gwendal is shocked that the village isn't recognized by Shin Makoku, and leads the attack on the bandits which targeted the village. In this duel, Gwendal sees Dunheely for who he really was, an old man, not the agile man he knew when he was younger. Dunheely later dies of old age, and Gwendal buries him near the tree where Dunheely told Gwendal what he was looking for. Years later while on a picnic with his family and Yuri, Gwendal admits that Dunheely was a larger existence than anyone else within him, because he hated that man.
Gwendal has no interest in love affairs, and even points out that issues coming from love affairs are beyond his abilities to influence. He does occasionally show care towards Greta, often being in charge of her while Yuri and Wolfram are in one of their adventures. Gwendal knows how to bake, as he does so for Greta when Günter, who would usually bake cookies for Greta, leaves the castle for a short journey. Greta comments that Gwendal's cookies look cute while Yuri comments Gwendal is good at baking as Gwendal's cookies are made into animal shapes, similar to the ones he knits. When Greta asks Gwendal if she is disturbing him while he is doing work, Gwendal says "Since it's you, I don't mind".
He is the commander of the demon army and often comes with reinforcements when Yuri finds himself in big trouble. He is also the one who does the paperwork and essentially runs the kingdom for Yuri. Often enough, when Yuri leaves the castle, Gunter is seen barging into Gwendal's office, demanding where Yuri is. Gwendal would usually look tired and suggest an idea which sounds appealing to Gunter, for example locking Yuri up in a room where no one will interfere.