King Thunder (サンダー王) is the ruler of Radaxian.
When Janken the Great invaded Radaxian, it was initially thought that Janken kidnapped King Thunder. After Janken was defeated in his castle by his son Alex Kidd, his other son Igul became the regent ruler. Later it was discovered that he was in fact alive on the planet Paperock. Alex went to the planet to rescue him, but discovered that King Thunder was there of his own accord.