Kyogre is named in mythology as the Pokémon that expanded the sea by covering the land with torrential rains and towering tidal waves. It took to sleep after a cataclysmic battle with Groudon.
– Pokemon Sapphire Pokedex
Kyogre is the mascot Pokemon of Sapphire, appearing on the boxart, and playing an important role in the plot. Its ability Drizzle was previously exclusive to Kyogre (but in Gen V, Politoed can get it from the Dreamworld). It’s based on the Leviathan, with inspiration from whales and orcas (though it’s still smaller than Wailord). Looking closely at it, it sort of has a derp face, haha.
Kyogre is the master of the seas, and the counterpart and rival of Groudon. You’d think with its Water type and Groudon’s Ground type, Kyogre has an advantage. But Groudon’s Drought ability combined with the fact that it learns Solar Beam by level-up can counter Kyogre.
In the anime, Kyogre was featured in an arc in the Advanced series, with Team Aqua and Magma facing off. It also had a small cameo part in the 9th movie (the Pokemon Ranger one featuring Manaphy).
Stat-wise, it has high Sp Attack and Sp Defense; its Sp Attack is tied with Palkia as the highest for Water types, and its Sp Defense is tied with Mantine for the highest of Water types. Its Attack and HP are pretty good too, due to its high base stat total (being a legendary and all). It’s the only non-Ice type Pokemon to learn Sheer Cold by level-up, and one of the few non-Ice types to learn Ice Beam by level-up. Kyogre can learn Water Spout by level-up. With its huge Sp Attack, and Drizzle to bring on the rain, its Rain-boosted Water Spout at full HP can do a LOT of damage.