Pokemon Uxie, Mesprit, and Azelf lore explained
The Pokemon series' legendaries Uxie, Mesprit, and Azelf have arrived in the Generation IV Pocket Monsters game. Often referred to as the Lake Guardians, the Lake Trio, or the UMA Trio, these three small Pocket Monsters hold immense power. They were tasked by Arceus with preventing the godlike Pocket Monsters Dialga and Palkia from battling each other and potentially unraveling space-time.
Since it's been quite some time since Pokemon Diamond/Pearl/Platinum or the remakes Brilliant Diamond and Shining Pearl have been in the limelight, it's worth examining the lore and backstory (from the core series games) behind the Lake Guardians. Understanding what makes them so important helps us understand their vital role in the Pokemon world.
Breaking down the lore of Uxie, Mesprit, and Azelf in the core Pokemon series
According to their inspiration, the Lake Guardians represent knowledge (Uxie), emotion (Mesprit), and willpower (Azelf). While very little is known about these three creatures, legends of the ancient Hisui region (which would eventually become the Sinnoh region of Pokemon Diamond/Pearl/Platinum) state that the trio was created by the deity Pocket Monster Arceus.
As Arceus created Dialga to control time, and Palkia to control space, it also purportedly made the Lake Guardians to help control them. When Dialga and Palkia clashed, the Lake Guardians could calm their rage or even directly control them when needed thanks to their immense psychic powers bestowed on them by the veritable god of Pocket Monsters.
Based on Azelf's Pokedex entries, the Lake Trio was birthed from the same egg. Their powers go well beyond protecting Hisui/Sinnoh as well, as the Lake Guardians are said to have bestowed humanity with knowledge, emotion, and willpower, and their collective name refers to the fact that they reside within Lake Acuity, Verity, and Valor within the modern-day Sinnoh region.
During the events of Pokemon Diamond, Pearl, and Platinum as well as Brilliant Diamond and Shining Pearl, the villainous Team Galactic manages to capture the Lake Guardians despite being battled at every turn by the trainer Lucas/Dawn (the canonical names for the player character based on gender). Team Galactic aimed to create Red Chains to hold dominion over Dialga and Palkia.
The three guardians head to the Spear Pillar, where Dialga and Palkia are engaged in battle. However, the Lake Guardians are incapable of subduing both Pokemon of Myth, instead calming one while the other continues to rampage. Giratina, the third creature of myth created by Arceus, senses the space-time imbalance and merges the real world with the Distortion World.
Cyrus, the head of Team Galactic, heads into the Distortion World, as the Lake Guardians give pursuit alongside Lucas/Dawn. The trainer encounters a puzzle within the Distortion world that requires the use of the move Strength to overcome, which the Lake Guardians assist with by using their powers before returning home and before Lucas/Dawn eventually seeks them out and catches them.
The Lake Guardians also assisted a trainer canonically named Rei/Akari (based on player choice) in the Legends: Arceus game, when the trainer was transported by Arceus from the current day into the ancient Hisui region. After being banished from Jubilife Village, Rei/Akari was tasked with creating the Red Chain to close the space-time tear above Mt. Coronet and repair the natural balance.
The Lake Trio tasks Rei/Akari with three trials to obtain Uxie's Claw, Mesprit's Plume, and Azelf's Fang, crucial components of forging the Red Chain. Rei/Akari successfully proves themself to the guardians, and the trio forges the Red Chain at Hisui's Shrouded Ruins, allowing Rei/Akari to mend space-time and ultimately come into content with the Pokemon of Myth and Arceus itself.
After successfully capturing Arceus, Rei/Akari returns to Hisui and begins collecting the godlike Pokemon's various elemental plates. When the trainer returns to Hisui's three lakes and catches the guardians, the stewardly Pocket Monsters reward them with the Draco Plate, allowing Arceus to change its type and that of its Judgment move into Dragon-type.
After the events of the Generation IV games and their remakes, Uxie, Mesprit, and Azelf have appeared elsewhere on occasion in other Pocket Monsters games like Sword/Shield, Ultra Sun/Ultra Moon, and Scarlet/Violet. However, they typically have no bearing on the overall story and are considered bonus legendaries to encounter and catch on players' adventures.