He is the prince of Finney, banished due to the fact he can't use Spell Arts.
He is recruitable at "Monster Extermination" PUB mission in Zweig, then, in Mules Weapon Shop.
Gustave, without cosplay is geared towards all weapons, due to his great Strength, Dexterity and Speed combination.
But his main fault is the fact that he can only use Roles, but he can still use Cards and Runes. This means no Shadow Servant to him unless by Night Party(Ginny Knights' last role) or Magician(Arcana Tarot).
Gustave's cosplay item, Gustave's Sword, changes him from his 14 year appearance to his 46 year one. He shall be slow as a rock, with 13 Speed, but 26 Strength and 21 Stamina.
With those setups it isn't a good idea specializing Gustave in Bare Fighting, Swords and Katanas, but Big and Giant Swords. But Gustave is better, in my opinion, without changing him to his 46 year old appearance. If you are going to kill him via "Disappearance of the Steel King!", give his sword to anyone, but don't equip it in Gustaf's last slot if he is using Katanas, Bare Fighting, Swords and magic, since Gustave XIII's mirror image will make all Gustaf's efforts with those will be in vain.