Onyx first appeared as a hologram in The Ancient Poké Ball, silhouetted with two other Admins, Sango and Agate.
Onyx reappeared as a hologram in HZ016, this time not silhouetted, where he commented that Spinel's reason for his mission's failure was the result of tunnel vision.
He made his physical debut in HZ024, where he accompanied Hamber and Sango to an old castle in Hammerlocke in order to meet up with Amethio, Zirc and Onia, who were pursuing a mysterious Pokémon that formed from Liko's pendant.
In HZ025, Onyx arrived at Old Castle alongside Hamber and Sango. Onyx had his Garganacl indiscriminately take out the Pokémon living at Old Castle before battling Liko and Roy. Thankfully, Captain Pikachu arrived at Old Castle to rescue them, Pikachu attacked Glalie and then took Garganacl out of the fight, before escaping when Sango suddenly had her Glalie use Self-Destruct. After the Rising Volt Tacklers had escaped, Onyx scolded Sango for her reckless actions, which not only caused unnecessary destruction but also caused both of their Pokémon to be defeated.
Onyx overall appears to have a serious and stoic personality, hardly showing strong emotions at times, although he can still get frustrated or irritated when insulted by others, such as his fellow admin Sango. He usually obeys the orders of his superiors such as Hamber without much issues. Onyx is along with Sango, known as "the troublemakers", being less disciplined and more aggressive than their fellow admins. Onyx has a ruthless and combative disposition when operating, as seen during the siege on Diana's castle where he not only disobeyed orders to standby, but also indiscriminately commanded his Garganacl to attack and take out every Pokémon in his path by having them inflicted with Salt Cure. Nonetheless Onyx has his own personal limits and considers even Sango's volatile actions to be reckless.