The King and Queen of the Southern Isles have 13 sons, with Hans being the youngest of them. The King and Queen most likely expressed favoritism towards the oldest sons. This left Hans feeling overshadowed, which, eventually, drives him to emotionally manipulate Anna in order to take over Arendelle. This would be the ultimate declaration of power and strength to Hans. He, as king, would no longer be overshadowed by his brothers.
Due to the Scandinavian setting of Frozen, the Southern Isles are most likely located somewhere in southern Norway, or possibly Sweden or Denmark, the latter of which is comprised mostly of islands. Because Hans is named after the Danish author Hans Christian Andersen, there is a possibility that he from a place representing Denmark. Also, since Arendelle is implied to be in Norway, Denmark would logically be The Southern Isles as Denmark is south of Norway in real life. This is further suggested in Frozen Fever, where, after Elsa accidentally sneezes into the horn and causes a huge snowball to be launched, it is shown travelling all the way to the Southern Isles, which is shown to be roughly where Denmark is.
Role in the CrossoverEdit
While Hans was facing his punishment, Pitch came up to him with the promise of giving Hans the throne and the chance to rise above his brothers, if Hans joins his cause and allows him and his allies safe pashas to and from and to use the island as one of their bases. Hiding in plain sight, without the people (and king and queen) of the kingdom having any knowledge of their presents.