Rayquaza with Lucius and its teammates in Alex's books
History
Pre-series
In ancient times, the Black Rayquaza travelled with an ancient adventurer named Lucius, along with 5 other Pokémon, Arboliva, GalarianMoltres, Lapras, Kleavor and Entei, together, these 6 Pokémon formed a group named the "Six Heroes", with Rayquaza as their apparent leader, alongside Lucius and the most powerful member. They all journed together, eventually meeting Terapagos in someway, who started travelling alongside them. After the group found Rakua and Lucius left to live there, the group scattered around the world. Somehow though, Rayquaza's ancient Poké Ball with it inside, ended in the sea and soon washed into an island's beach's shore, where it was found by Roy's grandfather a few years ago, being later gifted to his grandson.
In The Ancient Poké Ball, Liko's pendant's glow caused Rayquaza's Poké Ball to finally open during a battle between the Rising Volt Tacklers and the Explorers, it briefly battled everyone in the area, right before flying away in the direction of Paldea. Since then, the Rising Volt Tacklers, Roy in special, started seeking Rayquaza as it travels the world, along with the other members of the "Six Heroes", finding Arboliva and Moltres but failing to get in time to where Rayquaza was said to be.
Kabu meeting Rayquaza
It was mentioned in HZ009 when Liko's father Alex said that Black Rayquaza is heading towards Artazon. Then it was mentioned again by Brassius in HZ010 that Rayquaza is arriving at Arboliva Forest where an giant Arboliva lives.
In HZ012, 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 HZ020, Kabu revealed to have seen Rayquaza, soon after, the legendary Pokémon went towards the west of Motostoke.
Rayquqaza battling Amethio's Ceruledge
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 easily defeated them, before starting to attack the Rising Volt Tacklers as well, forcing Terapagos to transform into its Terastal Form and use Tera Starstorm against Rayquaza's Draco Meteor to protect its friends, which made Rayquaza leave once again.