Some fans have stated that Duke may be Kreutz's son. Upon further analysis of circumstances that surround Duke, I came up with the following evidence that may support this theory.

Weapon - Duke uses a weapon by the same name as Kreutz - Denta/Big Denta/Kamui. Actually, not only the name but the fact that they are both Two-Handed Swords sets it apart from Genshu's weapon, which has the same name but is a katana. One can argue that Kreutz gave his sword to his son.

Occupation - Duke is a mercenary within the Harmonian Border Guards, just like his father. After the end of the Gate Rune Wars, Kreutz simply vanished. The Harmonian Border Guards is well known for being an organization that allowes outcasts, misfits, and those with a "past" to enter their group as long as they are able fighters.

Nicholas - Nicholas is Duke's loyal protector, who has served Duke's father. The fact that Duke's father is able to have subjects with such lasting loyalty points towards a strong possibility that he was a person of high rank. Kreutz was a general in the Scarlet Moon Empire, and based on Nicholas's age, it is not far fetched to think he participated in the Succession War as well as the Gate Rune Wars under the command of Kreutz.

Gau - Gau is said to have come from Toran's Kobold Village. He also joined Duke because he saved Gau's life. Connecting the two, one can possibly say that Duke saved Gau's life back in Toran. Otherwise, there would be no good explanation why a kobold from Toran ended up in Caleria.

Elaine - Elaine uses lipstick that is produced in the Kunan Region in the Toran Republic. Elaine and Duke knew each other at least in IS 460 (SOurce: Card Stories) -- if Duke was in Toran back then, it would make sense that Elaine would have also been there, and learned to use lipstic that is made in Toran. If that is the case, this evidence will support the claim that Duke was in Toran along with his father.

Age - With 20 years between Kreutz and Duke's birth, Kreutz can easily be Duke's father.