Lugia
Gender: ♂
Birth Name: Lugia/Ryūjin
Personality: Prideful, Serious, Impatient, Sympathetic, Protective…
Favorite color(s): Silver, Ocean Blue, Magenta, White
Time of Birth: approx. 2 billion years ago?
Time of Death: (INVALID)
Time of Rebirth: (INVALID)
Form before Rebirth: (INVALID)
Hair Color: Lavender
Favorite Animal(s): Mosasaurs, Birds, Dinosaurs, Plesiosaurs
Parentage: Arceus (Biological father), Articuno (Biological daughter), Zapdos (Biological son), Moltres (Biological daughter), Otohime (Foster Daughter)
Cause of Death: (INVALID)

Ho-Oh
Gender: ♀
Birth Name: Ho-Oh/Fenghuang/Achiyalabopa
Personality: Caring, Controlled, Calm, Sympathetic, Cheerful…
Favorite color(s): Has no least favorite…
Time of Birth: approx. 2 billion years ago?
Time of Death: (INVALID)
Time of Rebirth: Unknown, cycles through at various and random points in time…
Form before Rebirth: (INVALID)
Hair Color: Blonde with Red Highlights
Favorite Animal(s): Has no least favorite…
Parentage: Arceus (Biological father), Raikou (Adoptive son/Creation), Entei (Adoptive son/Creation), Suicune (Adoptive daughter/Creation)
Cause of Death: (INVALID)

Biography;
Ho-Oh and Lugia were twin children to Arceus to safeguard the Sky and Sea on Planet Earth, Ho-Oh came to be known as Fenghuang to the Chinese, and Achiyalabopa to the Pueblo people, while Lugia went by Ryūjin to the Japanese…

Ho-Oh was very kind, and gave sympathy to the mortals, causing her to be beloved by all, and in her human guise, won the hearts of practically every man who gazes upon her. Though genuinely charmed by many of the proposals and proclamations of love she receives, she politely and sweetly turns all of them down, even directing them to someone else to make it up to her admirers. Ho-Oh has a tremendous soft spot in her heart for children, and individuals who are timid and/or shy, seeking them out to play with. Her kind personality was a result to her close friendship with an individual known to the mortals as, Jesus Christ, taking inspiration from him and often offers to help him perform miracles. She may be kind and even sympathetic most of the time, Ho-Oh has no tolerance for anyone who treats life like a toy that they can throw away, and is very misleading to her power and skill, as she is incredibly gifted in martial arts, able to face against any opponent she faces. She is one of the few, true immortals in the universe, but follows a death-like procedure in a cycle of life, death, and rebirth, she also can use her flames for all kinds of strange and unique abilities, like healing…

Lugia was quite bitter at times, and in modern times, lives with immensely deep regret over his actions in nearly ending the human race, by creating the Great Flood of biblical times. He was good friends with the archangel, Ryūjin, and took his name and fostered his daughter, Otohime, when he died fighting the demon, Seta, nicknamed the Dragon-Devouring Centipede. He once fell in love with a human girl, and was one of the few Legendary Pokémon to have mortal children, in the form of two daughters and a son, who would become Articuno, Zapdos, and Moltres. A demon heard of Lugia falling in love with a human, and killed her as a result, unaware of the girl’s children with Lugia. Lugia spent countless days searching for this demon that murdered the love of his life, growing angrier as his efforts constantly failed. His grief would intensify when years later, the same demon who killed his wife, had killed his children, using humans to do the dirty job. His great strength is utilized by his high-power attacks and incredible defenses, holding a unique power to create a healing rain that can restore burned wastelands into lush fields…

Ho-Oh came to love her nieces and nephew, and was just as heartbroken when she saw them killed, by making a deal with the Angel of Death, she had them reborn as Legendary Pokémon, Articuno, Zapdos, and Moltres. Since the demon murderer had escaped once again, Lugia was overcome with regret on his actions, leaving only the humans, animals, and Pokémon upon an Ark built by Noah. When Ho-Oh told Lugia that the spirits of his children were within the three birds she presented, he was forever grateful and had adopted his reborn children, vowing to protect them with all his might…

When both Ho-Oh and Lugia came to rest upon the two towers in Kyoto, they oversaw the humans and were respected and even worshiped. When a terrible storm and an oncoming war was about to strike, lightning bolts struck the Brass and Tin Towers, Lugia fled to the ocean as the bolts set his body ablaze. Ho-Oh did not want to flee and using her most powerful technique, single-handedly ended the war and somehow generated a downpour. Feeling very sorry for her brother, Ho-Oh decided to leave as well, but not before she saw the bodies of three Pokémon that were killed by the Brass Tower collapsing on top of them. She was overcome with pity as she revived the three Pokémon into Raikou, Entei, and Suicune, then left, deciding to seek out a human with a pure heart of gold, a soul of silver, and a consciousness as clear as crystal, to fight alongside, eventually convincing her brother via telepathy, to do the same. Whereas Ho-Oh saw goodness in humans and all sentient life and strives to guide them to a morally righteous path, Lugia grew bitter of sentient lifeforms, especially humans, resenting them silently as his father, and his lord and master, kept him from destroying humans. Lugia’s cold heart to humans eventually softened when he met a human child that changed his mind, and now fights alongside the child, a Pokémon Trainer…

4869231 These headcanons are amazing!

4870493 Yeah, a good majority I have planned and thought out, but there are quite a few I haven't thought out properly. Here are the ones I'm having a little trouble with;
-The Lati Duo (Having trouble finding a real-world legend to tie with them)
-Jirachi (Not exactly enough details on this Pokémon)
-Heatran (in terms of it being either gender, haven't fully thought it out yet, except for background)
-The Swords of Justice (Having difficulty writing a good enough background for them)
-Diancie (Like Jirachi, not a lot of details...)
-Volcanion (No real information has been given yet...)

I'm holding off on Xerneas, Yveltal, and Zygarde because I want to know how the three tie together, and the next Gen 6 games might answer that for me...
If you would like to help out, I'd be happy to accept it...