Groudon
Gender: ♂
Birth Name: Groudon, Behemoth
Personality: Impatient, Short Tempered, Protective, Serious, Prideful, Competitive…
Favorite color(s): Molten Red
Time of Birth: approx. 8 billion years ago? Approx.
Time of Death: (INVALID)
Time of Rebirth: (INVALID)
Form before Rebirth: (INVALID)
Hair Color: Crimson Red with Gold Highlights…
Favorite Animal(s): Big Cats, Elephants, Crocodiles, Bears, Theropods…
Parentage: Arceus (Biological father), Dwarves & Trolls (Worshippers), several fire and earth-based aliens (Descendants? Worshippers?)
Cause of Death: (INVALID)

Kyogre
Gender: ♂
Birth Name: Kyogre, Leviathan
Personality: Impatient, Short Tempered, Protective, Serious, Prideful, Competitive…
Favorite color(s): Oceanic Blue
Time of Birth: 8 billion years ago? Approx.
Time of Death: (INVALID)
Time of Rebirth: (INVALID)
Form before Rebirth: (INVALID)
Hair Color: Deep Blue with Vibrant Red accents…
Favorite Animal(s): Mosasaurs, Sharks, Crocodiles, Plesiosaurs, Ichthyosaurs…
Parentage: Arceus (Biological father), Atlantians (Worshippers), several water-based aliens (Descendants? Worshippers?)
Cause of Death: (INVALID)

Rayquaza
Gender: ♀
Birth Name: Rayquaza, Ziz
Personality: Short Tempered, Flirty, Intelligent, Caring, Proud, Competitive, Bloodthirsty, Mature, Expressive, Mischievous…
Favorite color(s): Emerald, Forest Green, Molten Red, Oceanic Blue, Earthy Brown, Sunshine Yellow, Vibrant Orange, Royal Violet…
Time of Birth: 840 million B.C.E.? (Minimum age, implied to be far older)
Time of Death: (INVALID)
Time of Rebirth: (INVALID)
Form before Rebirth: (INVALID)
Hair Color: Jet Black or Deep Emerald Green (depends on her ‘stylish’ mood)
Favorite Animal(s): Doesn’t have a least favorite, but prefers dinosaurs…
Parentage: Arceus (Biological father), Mayans (Worshippers)
Cause of Death: (INVALID)

Biography;
Groudon, Kyogre, and Rayquaza are considered three of the oldest beings in the universe, having said to have been around since the first planets formed. It is unclear who the eldest among the three legends is, even among themselves. This often causes heated arguments, and then violent clashes later between Groudon and Kyogre, Rayquaza however, doesn’t really care who is older or younger due to her maturity. Groudon and Kyogre fight worse than cats and dogs, as in the words of their sister, Rayquaza, “They always find something to fight or compete over. From who gets the girl, to something as stupid as who gets the last piece of popcorn...” Rayquaza gets along fairly well with either Groudon or Kyogre, but gets very annoyed, and sometimes depressed when they bicker and fight like there’s no tomorrow, as the two brothers are fond of their sister. More often than you think, Rayquaza frequently has to fight and viciously beat her brothers and scold them about their constant quarreling, as in more recent decades, Groudon and Kyogre grew to (sometimes) fear fighting each other while their sister is watching them. Their lifespans are so incomprehensibly long, that there are million year gaps where their lives could easily be up for debate on what they did…

Groudon boasts immense physical strength and will, and was once thought to have been the Greek Titan, Atlas, but Groudon had disproven that he and Atlas, though a real entity, were not the same being as the humans believed. He is, however, the true identity of the biblical monster known as the Behemoth, described as being more terrifying and powerful than the monster he was said to be, as a strike from his massive hands can create fissures in the crust large enough to swallow islands the size of Madagascar, and swing his tail with enough force to generate shockwaves strong enough to snap all but the sturdiest of trees (hence ‘a tail that moves like a cedar’). But, unlike the Behemoth, Groudon never set foot in Hell, and despises the Devil and his followers, has a deceivingly small appetite, and is a terrible singer compared to the Behemoth’s descriptions of its singing voice (but a fairly decent singer to mortal ears). Compared to the modern day, Groudon was especially violent, almost animalistic, in the past with his true, original power, or known today as his Primal Reversion. Many demons and creatures of the night hold great fear over Groudon, as his presence invokes very intense, ferocious sunlight, even at night when the sun is not in the sky, as many of them are weakened or killed in sunlight…

Kyogre also has tremendous strength and will, and much like his brother, was once thought to have been the Greek God, Poseidon, but this was disproven by both Kyogre and Poseidon himself. But, Kyogre is the true identity of the terrifying monster in the bible known as the Leviathan, or at least, took on the original Leviathan’s form when he succumbed to wickedness. Due to Kyogre’s ferocity, the Leviathan’s legend grew more terrifying, with several descriptions being written in response to Kyogre taking the name of the Leviathan for himself, even creating a bodily form that frightens the most powerful beings in the universe, and drives the most powerful demons of Hell to madness of fear at the very sight of this horrifying form. Kyogre seems to take great pride in being the Leviathan, especially when he states that he and his superior destroyed the original Leviathan in fire and lightning, especially to the lost kingdom of Atlantis as they worship him and his lord. Demons and monsters always dread and sometimes flee when Kyogre comes to battle, since the rain he summons purifies the wicked energies of the forces and creations of Hell. His ferociousness was heightened long ago, like his brother, border-lined an animalistic savagery, and even in the modern-day era, Kyogre (and Groudon) feel that, sometimes, humans and other mortal beings are inferior and weak, but have some admiration in their resolves. This would spare some humans from their wrath, but not all…

Rayquaza is a very obviously different from her brothers in strength and personality. Aside from being intelligent and quite emotive, she also has a good, yet strange sense of humor. She was once thought to have been the Greek God, Zeus, but she says, with a hint of arrogance, that Zeus grovels at her feet to not anger her and that she is the thunder god’s boss. She gets bored frequently due to her long life, so she created an unusual hobby by subtlety changing into a man’s ideal woman (but the changes are almost never noticed at first glance until later), then flirt with them to see their reaction an puts it down on a ‘chart’. A long time ago, Rayquaza once fell in love with an alien, but fate had other plans when her lover perished to enable freedom on his planet. Rayquaza seeks her lover out every so often since she heard he had been reincarnated somewhere in the universe, but knows not where. Tasked with not only safeguarding the skies, Rayquaza is also tasked with keeping both Groudon and Kyogre’s battles from growing out of control. However, one day, Groudon and Kyogre fought especially violently against each other that threatened to destroy the entire planet, Rayquaza intervened and the three siblings fought for days in a titanic clash that could be felt far out of the solar system. Rayquaza won, but put both her brothers in a coma-like state, and realized she nearly killed them to end the fighting, causing her to fall into a state of depression and self-loathing for hundreds of years. Rayquaza recovered, but had left her with no tolerance of in-fighting between friends and especially family members, causing her to feel greatly saddened and upset. Caring deeply for her family and friends, she often takes serious risks to spare their lives…

All three exhibit extraordinary talent in fighting, but Rayquaza is the most skilled, utilizing and mixing various skills from multiple universes to make them her own, possessing the necessary intelligence to use such a talent, like combining the use of ki into her arm-swords into plasma blades. Though it is not known who the eldest of the three siblings is, the most mature is without a doubt, Rayquaza. Groudon is pretty famous for striking with surprising speed and power, despite not being very agile. Kyogre has a great mastery over attacking at a distance, and could utilize long-range weapons with surprising accuracy. Rayquaza’s weak point is her defenses, but more than makes up for it by being fast and offensively powerful. The three eventually come to bond with three friends, two rookies and a veteran, all of them are Pokémon Trainers…

5032138 Very interesting, I personally would've considered Kyorge to be feminine and Rayquaza to be masculine, but the headcanon for this trio is as good as your previous ones.

5032458 I thought it be more amusing and unexpected if Rayquaza was the girl of the trio, also a head's up, Rayquaza has a serious pet-peeve about being thought of or mistaken as a male...At first, I did think of Rayquaza as a boy, but it was short-lived when I saw numerous fan art pieces of a humanized Rayquaza as a girl, and it became so much cooler and awesome than a boy...

I made a form/disguise of Rayquaza as a Mobian (whatever Sonic the Hedgehog's race is called) on DeviantArt that you can check out, since I put more details about her there: Follow this link...
It gives you an idea of how I think she usually looks like...

I am making her a human form/disguise and one of her as a pony (Haven't decided which type, but do not want her to be an alicorn)...

I think it's a little late to say this, but you should expect some of the other Legendary Pokémon to have unexpected circumstances regarding their genders or personalities...