An alien criminal from the planet Aryll who comes to Earth in an attempt to use a highly intelligent Earthling in his bid to take control of his home planet. Members of the Aryllian race all possess some degree of extra-sensory perception powers (in Klaxxu's case, this manifests itself as a form of telekinesis), and can be poisoned by the traces of helium within Earth's atmosphere, though periodic ingestion of a chemical solution made from formaldehyde will counteract the effects of helium on an Aryllian's system.
Arriving in Smallville, and posing as Smallville High science teacher Philip Mason, Klaxxu uses a series of tests and exams to determine a suitably intelligent candidate to be used in his plot. Selecting young Clark Kent, the secret identity of Superboy, and using a device called a Mind-Melder to take control over the teenager's mind, he convinces Clark that he is in fact another refugee from the planet Aryll named Bovra, who Klaxxu had hidden away years before at Smallville Orphanage.
Partially under Klaxxu's control, Superboy fights to regain control over himself from the manufactured "Bovra" personality. After flying through a balloon filled with helium, and thus destroying Bovra, Superboy flies back to the Mason residence to apprehend Klaxxu, which he does by blowing a gust of helium in the criminal's face, rendering him unconscious. Soon after, Superboy returns Klaxxu to Aryll and places him in the custody of the Aryllian authorities (SF No. 197/1, Oct 1979: "Superboy's Split Personality").