Klaus von Hertzon (クラウス・フォン・ヘルゼン, Kurausu Fon Heruzen?) used to be the second-best Bakugan player before he lost to Dan Kuso, which caused him to drop down to fourth place. Klaus was first seen in Bakugan Battle Brawlers and became a supporting character in Bakugan: New Vestroia. He is an Aquos brawler with a close bond with his Guardian Bakugan, Sirenoid. Initially an antagonist due to Naga's brainwashing, he grows spiteful towards Alice Gehabich, Masquerade's unwitting host.