What constitutes an 'active' legend:
Active legends are those who contribute to the story/have bios. While all legendaries in Pokeau are important, only a select few are actually developed and active. Additionally, active legends usually have their lore fleshed out more, and are mentioned more in-universe as a result.
This page will only cover the legends and not their emissaries - more info about them can be found on their profiles.
THE COOLER LEGENDS
Or, in simpler terms, the overseeing gods (and any who might happen to be related). Often living on a higher plane and watching from above, these legends are all-powerful beings capable of doing... whatever they want, essentially. For them, the term 'emissary' doesn't quite fit - they are the legendary itself.
The creator and source of final judgment in the main universe. Responsible for creating the original ancestor mew along with all other legends, they hold the utmost power of the realm and watch silently from the hall of origin.
LUNALA & SOLGALEO
Previously represented by Skymother
Whether or not Skymother created Ultra Space is unclear, but she did have complete control over it, and was revered as an angel by the sapient species of the land. She was responsible for creating humans and could change her form to better protect her universe.
COSMOG
Represented by Nebby
Cosmog is the child form of Skymother, created when she expended all her power to save Ultra Space from destruction. While they are technically capable of becoming Lunala or Solgaleo again, they're currently too unstable to get anywhere close. They only hold a fraction of Skymom's power, none of her memories, and is prone to mini-supernovas which cause them to lose all their current memories and start from the beginning again.
NECROZMA
Represented by Ramiel
Nicknamed 'the blinding one' and having their true name lost to time, the populace knows them as Necrozma. They're the child of Arceus and Skymother, who suffered great physical and existential consequences of being a divine child across universes. While originally a being of light, Ramiel soon turned to violence over hatred for Arceus - they took all of Ultra Megalopolis's light and threatened to destroy Ultra Space as a whole. It wasn't until Skymother used all her power to contain them that their rampage ended.
EVERYONE ELSE
RAYQUAZA
Represented by Emmy
Known as not only the peacekeeper of the Hoenn ancients, but as the peacekeeper of legends around the world. Rayquaza is said to work closely with the people of Hoenn and is believed to be directly responsible for contributing to many of Hoenn's historical events. Most notably being the destruction of meteors that threaten the region, if not the world's wellbeing every 1000 years. Rayquaza is also the primary god of Draconids, and is considered to be the ultimate dragon by the more modern Draconid counterparts.
KYOGRE
Represented by Sapphire
While prioritizing Hoenn, Kyogre is the god of the oceans all around the world. They can control the water of the ocean and elsewhere at will, and is known to create catastrophic monsoons and hurricanes when enraged. While it's track record in history and legend might not be the most positive, Kyogre is also responsible for maintaining the health of the oceans and it's denizens, and has been known to prevent hurricanes when in a good mood.
LUGIA
Represented by Basil
look man i even checked bulbapedia for this and they straight up admitted lugia does jack shit, it's all about ho-oh here, so like at least i have an excuse to have no idea what lugia does. anyways. Lugia is supposedly the guardian of the Whirl Island and coastal Johto, but in recent history, they've caused more problems than anything else because the emissary RNG sucks I guess. Maybe Basil can set the record straight :/
HO-OH
Represented by Kaguya
The revered protector of mainland Johto, and an iconic figure for the region itself. Seeing Ho-Oh is said to grant wishes, and untold power is said to be in store for those who come into possession of one of its rainbow feathers. Ho-oh was a vital part of Johto's history, and would gift it's knowledge and grace to those it deemed pure of heart. Despite an extended absence after war, Ho-Oh is still worshipped regularly by those who hope to see it return.
DIALGA & PALKIA
Represented by Ridley and Pearl respectively
Being connected to Arceus directly, these two are some of the more mysterious of the legends, rarely even appearing in the world and preferring to stay in their pocket dimensions. They were created to control and keep balance of time and space respectively, and despite being so rarely seen, these important roles made them paramount in Sinnoh's religion (though not to surpass Arceus, of course), as well as beliefs elsewhere.
CRESSELIA
Represented by Aster
Though it might be a minor legendary, Cresselia has an eerily large following in Sinnoh. Its primary role is the goddess of dreams and keeper of the dreamscape, and it's said that hanging a lunar wing from Cresselia over your bed protects from nightmares and allows Cresselia to bless your dreams. It's especially popular among the 'cosmic psychic weirdo' type, and one of the largest psychic-specialist gatherings happens to be a festival in honor of Cresselia, located in Canalave.
RESHIRAM
Represented by Angie
One of the two royal dragons of Unova, Reshiram is known for siding with the brother who believed in the truth. Since that defining moment over 2,500 years ago, Reshiram has continued to appear in two-sided conflicts in the region and will side with the individual who it feels personifies the truth of the situation. The two dragons symbolize duality togther, and it was often through their frequent fighting that a compromise of truth and ideals was realized.
YVELTAL
Represented by Aurelien
The embodiment and god of chaos itself. Yveltal is the force that balances Xerneas's ability to create life, and boy, it does it well. Throughout Kalos's history, there's no shortage of selfish kings who seek to exploit Xerneas's power - who only end up experiencing Yveltal's wrath. While it remains mostly confined to the Kalos region, the effects of the destruction caused by Yveltal are felt all over.
XERNEAS
Represented by Amalthea
Standing opposite to Yveltal is Xerneas, the god of life. Many seek out Xerneas for its ability to grant immortality, but in reality, Xerneas is quite reclusive - opting to use its power to maintain the health of the world around it. Xerneas is also seen as a revolutionary icon of sorts in Kalos, after a woman who claimed to be blessed by the god led a revolution against a corrupt king.
JIRACHI
Represented by Apollo
Known to humans as a sleepy god of wishes, Jirachi is actually quite more active than mythology implies. It can sense meteor showers and comets, as well as the wishes people make to them. Using this sense, it grants wishes to people it deems deserving. If it uses up too much energy, however, it enters a long slumber.
CALYREX
Represented by Francis
Calyrex is a Galarian god of harvests, and possesses the ability to powerfully manipulate plant life. It's capable of sprouting plants from nothing, as well as relocating entire forests worth of plant life. It's also been a symbol of the Galarian royal family for centuries, though active worship and belief of Calyrex dropped sharply after Galarian royalty was disassembled.
MANAPHY
Represented by Marie
As the guardian of coral reefs and tropical waters, Manaphy is a lesser god that has a better track record than Kyogre. It can manifest underlings known as Phione, who act as buoys and relay information on the water's health back to Manaphy. It also has the ability to purify tainted water.
(ill make a shiny sprite later)
CELEBI
Represented by Megumi
Celebi is known as a lesser guardian of the forests, though what makes it unique is its ability to view the past. It's seen as a messenger for Ho-Oh, and despite being a lesser god, is highly important to Johto's religion. It's not uncommon to find Celebi shrines littered across forests in Kanto and Johto.