Commander Leo is a minor villain in the non-canon game Ys IV: The Dawn of Ys. His role is changed in the other versions of Ys IV.
Ys IV: The Dawn of Ys
In Dawn of Ys, Leo acts as one of the generals of the Romun Empire, being stationed on a base near Arieda Village. He imprisons Adol when the hero approaches their base, and is later seen conspiring with Gruda to find the lost city within the Forest of Celceta. After the city is unearthed, Leo goes along with his men to the city in search of a legendary treasure, which ends with he all of his men being killed on the many traps in the place.
Ys IV: Mask of the Sun
In Mask of the Sun, Leo's role is taken by a unnamed Romn General, who has similar objectives and is also conspiring with Gruda. Later on in the game he attempts to take over the town of Comodo, where he is confronted and defeated by Adol.
Ys: Memories of Celceta
Leo returns in Memories of Celceta, where he is changed into a comic-relief character. This version of Leo is not villainous, rather acting as a incompetent rival to Adol who is tasked by the Governor General Griselda to map the Forest of Celceta and later to track down Gruda.
Ys Origin and Ancient Ys Vanished