Jack is a member of the Gallia Knight army along with his brother Bleu and their sister Jeanette. The Gallia Knight army, a group Tabitha once belonged to, is known for their "dirty duties" as Agnes put it.
Jack and Bleu stole the Founder's Round mirror, and captured Louise in an unknown treasure room within Romalia. Saito found the two thieves, but Bleu escaped, taking the relic with him. Jack fights Saito head to head proving his skill in magic and combat skills.
Jack is an earth mage capable of summoning golems just like Guiche, except unlike Guiche he also shows skill in non-magic combat. Specifically, he is shown creating a chained mace from a small piece of broken wall. Adept in fighting he proves a match for Saito's Gandalfr powers with his weapon skills. Jack is capable of using magic without directly using his wand, when making his mace, jack needed only to touch the fragmented rock in order to transform it.
Mage guards are shown escorting him away from the grounds after Louise's explosion had stopped him from delivering a final blow to Saito.