The Shake King comes to us from Wario Land Shake It!. His ulterior motive for going after a piece of Tabuu in subspace was that he wanted revenge on Wario for making him lose the bottomless coin sack. When Tabuu offered him money and power in exchange for his loyalty, Shake King's lust for revenge was overshadowed by his lust for treasure. As such, he will remain loyal to Tabuu as long as Tabuu pays him.