Rayquaza with Lucius and its teammates in Alex's books
Pre-series
In ancient times, the BlackRayquaza travelled with the Ancient Adventurer, Lucius, along with his other five Pokémon, Arboliva, GalarianMoltres, Lapras, Kleavor, and Entei, forming together a group named the "Six Heroes", with Rayquaza as their apparent leader alongside Lucius. They all journeyed together, eventually meeting Terapagos, who started traveling alongside them. After the group found Lakua and Lucius left to live there, the group scattered around the world. Somehow though, Rayquaza's Ancient Poké Ball with it inside, ended up in the sea and eventually washed up on the shoreline of an island, where it was found by Roy's grandfather a few years prior to the series, being later gifted to his grandson.
In The Ancient Poké Ball, Liko's pendant started to glow in conjunction with Rayquaza's Poké Ball, which finally opened during a battle between the Rising Volt Tacklers and the Explorers. The ShinyLegendary Pokémon briefly battled everyone in the area, right before flying away in the direction of Paldea but not before staring at Liko. Since then, the Rising Volt Tacklers, Roy in particular, started seeking Rayquaza around the world, forming bonds with the other members of the Six Heroes like Arboliva and Moltres, but failing to get in time to where Rayquaza was said to be.
In The Future I Choose, Liko and Roy found out about the Six Heroes's story in Alex's books, knowing now that Rayquaza was Lucius's main and most powerful Pokémon.
In HZ032, the Rishing Volt Tacklers reencountered the Black Rayquaza in the sea right after Lucius's Lapras joined them. In HZ033, Rayquaza was suddenly attacked by Amethio's Corviknight and Ceruledge, but it easily defeated them, before starting to attack the Rising Volt Tacklers as well. This forced Terapagos to transform into its Terastal Form and use Tera Starstorm to counter Rayquaza's Draco Meteor shower to protect its friends, which left Rayquaza amazed before it left once again.
In HZ043, Rayquaza was mentioned by Coral, revealing that the Explorers were planning to capture the Sky High Pokémon. In the next episode, Spinel put his plan into motion, which involved trapping Friede to buy time while Coral, Sidian, and Chalce fought Rayquaza and attempted to catch it. With the help of the Rising Volt Tacklers, Rayquaza managed to free itself from the trap created by the Explorers, but thanks to a reckless Self-Destruct used by Coral's Glalie against Rayquaza, it was sent crashing into the Brave Olivine, damaging the ship.
In HZ045, most of the Explorers retreated, aside from Amethio. The Black Rayquaza was confronted by Liko and the others and readily accepted Liko's offer to battle it. Amethio later joined the fray and teamed up with Friede, with them Terastallizing Ceruledge and Charizard, respectively, but both were eventually defeated by a devastating Dragon Ascent. Liko and Roy confronted Rayquaza next, but despite Fuecoco learning Flame Charge and Sprigatito evolving into Floragato, they were unable to do much against the Sky High Pokémon. Before leaving, the Black Rayquaza locked eyes with Liko again, seeming to take note of the resemblance she bears to its partner, Lucius.
Personality and characteristics
Not much is known about the Black Rayquaza. It is the most powerful of the "Six Heroes" and apparently, their leader alongside Lucius. It appears to be very aggressive and for some reason, prefers to fly away from the ones seeking it, including its former teammates and Terapagos. It has also shown some kind of interest towards both the Pokémon and Liko, probably due to the girl's family being linked to Lucius somehow and due to it knowing Terapagos from the past. Diana theorizes that it wants to see its true power.
In its appearances, the Black Rayquaza possesses a brilliant, crystallized appearance, heavily similar to a Terastallized state, but lacking a Tera Jewel on its head.
This could indicate a bigger connection between Rayquaza and Terapagos, not only both used to be Lucius's Pokémon, but it was also Terapagos in its dormant state who awakened Rayquaza from its Ancient Poké Ball.