Trio master

A Pokémon that is a trio master is one who, in some way, oversees a legendary trio of Pokémon.

The trio master and the trio itself may relate in different ways. For example, Lugia is the trio master of the legendary birds; it is the only one who can quell their fighting when Lawrence III captures one of them in The Power of One.

In the games, the trio master sometimes will and sometimes will not require that the members of its legendary trio be captured beforehand. In Pokémon Crystal, Ho-Oh cannot be captured until all three of the legendary beasts have been, and in Diamond and Pearl, Regigigas will not awaken unless the player has the legendary golems in his or her party. Lugia and Arceus, however, do not seem to have this requirement, nor does Ho-Oh in Pokémon Gold and Silver and Rayquaza in the Generation III games. However, in Pokémon Mystery Dungeon, the legendary birds must all be befriended before the player can enter Lugia's dungeon, and before encountering Ho-oh, the three legendary beasts must contribute their power to create the Rainbow Wing. In Pokémon Colosseum in order to get Ho-Oh, the player must first capture and purify all the Shadow Pokémon, which includes the legendary beasts, as one of the two requirements.

Trio masters' relation to their trio

Trio master	Legendary trio			Relation
Lugia	Articuno	Zapdos	Moltres	Lugia rises to quell the fighting of the legendary birds in The Power of One, and is the only one who is able to do so.
Ho-Oh	Raikou	Entei	Suicune	After the three legendary beasts perished in the fire at the Brass Tower, Ho-Oh revived them.
Rayquaza	Groudon	Kyogre	Rayquaza	Rayquaza may internally be the master of the weather trio, primarily because it had quelled the fighting between the other members of its trio in Pokémon Emerald.
Regigigas	Regirock	Regice	Registeel	When the three legendary golems are brought to Snowpoint Temple, Regigigas will awaken. Regigigas' Pokémon Platinum Pokédex entry also claims that it created itself out of rocks, ice, and magma.
Arceus	Dialga	Palkia	Giratina	According to legend, Arceus created Dialga and Palkia. It's also stated in Pokémon Platinum that Giratina was created alongside Dialga and Palkia. All four legendary Pokémon will battle in the 12th Movie, Arceus: To a Conquering Spacetime.
	Uxie	Mesprit	Azelf	Arceus is said to have created the lake trio whilst creating the universe.