Onyxwing is a wild Archeops that René befriended while she and Émile were in an unknown area that she had never seen before. It was filled with Archen and Archeops.
Onyxwing stated that he had never seen humans before. He also told them not to tell others about that place.
In Lost, Roxie catches Onyxwing and Marlon catches his new friend, Icetooth and both are transferred to René's lab. He is reunited with her. She asks the two Pokémon if they want to go outside and they say yes.
It is unknown as to what happened to him before Protection.
He states that he had never seen humans before, and that he wants the area to be a secret. He wasn't seen again, but it's possible he was an Archeops that was watching Émile and René as they left.
It is unknown as to what happened to him before Lost.
In Lost, Marlon manages to grab his tail. Marlon and Roxie are shocked by his appearance. He tells them that he has seen humans before, which causes them to be confused. He wonders if they'll "mate" on the beach too, and the duo promises they won't. He then shows them to some water.
He then finds them again, wondering if they need firewood. Marlon asks if Roxie wants to take off her shirt, and when Onyxwing returns, he thinks they "did it". Marlon then tells him that their relationship is platonic, and Onyxwing doesn't know what that means. He then tells the ancient bird what it means. Onyxwing then gives them some firewood, and flies off, but before so, tells him if they want anything else, that he'll help.
In chapter 6, he appears before Roxie and Marlon, wanting to help them. He wanted Marlon to command him in battle. Marlon commanded him to use Dragon Claw, but he ended up using Shadow Claw instead. He then tells Onyxwing to use Dragon Tail to counter Landorus' Stone Edge. Onyxwing eventually defeats Landorus with an Aqua Tail. Marlon then tells him to take a break. After Marlon defeats Landorus, he then shows them where they can get some food.
In chapter 8, Onyxwing asks what fish tastes like. Marlon does his best to describe what they taste like, he states that he's seen dead fish on the beach, and hadn't had the heart to touch one. He then tells Marlon that his species don't get parasites often, except when they're sick and can't preen themselves, they will let their mates do so. He then follows Marlon back to Roxie. He quickly befriends Icetooth. While Roxie and Marlon gather berries, Icetooth and him catch fish. When the duo comes back, Marlon notices burns on him. Roxie says "ouch" and walks up to him, using a super potion on him. He is grateful for Roxie's super potion, and tells her that he feels better because of it.
In chapter 9, he agrees to help Marlon with faking his death. In chapter 10, he comes out of hiding, and asks Roxie if she could catch him, along with his friend, Icetooth. Roxie says that he'll be sent to René's lab. He says he doesn't care and he immediately taps a pokeball with his beak. Marlon catches his new friend, Icetooth. The two are sent to her lab. When René lets the two out, Onyxwing immediately tackles her to the ground, telling her that he missed her. She then asks them if they want to go outside. The two are not seen again after chapter 10.