Is actually the human form of Necrozma. While most legendaries of that status have adult forms, the damage done to his body and brain by those of Ultra Megalopolis have given him permanent damage, giving him a child's mentality, and so child's body. Necrozma's always been one of the more... volatile legendaries, with it's ability to steal light, but the damage and pain physically and mentally that it was subjected to made it actively hostile, leading to its imprisonment in the tower... which did not improve its state at all. A year or two before the Alolan plot begins, it breaks out and blindly lands on Earth in its human form, confused. Sensing that a Cosmog is in the region, it decides to take advantage of the situation by pretending to be a lost human who fell through the wormhole, while making a plan to steal it or the sun...
Staying as a human does give him the benefit of not being in constant severe pain. Though he's still weak, sleeps frequently, and has extremely bad eyesight (though he doesn't seem that bothered with it, which gives the adults around him the impressions of developing psychic abilities). He's naturally nocturnal, but is prevented from being so due to everyone around him having normal sleeping schedules. Still, he does do sneaking at night sometimes.
Being found in Iki Town, he ended up being fostered by Hala. Hau very much enjoyed having a "brother" his own age, and Eleao comes off simply as a sweet boy with some issues to the majority of people. Considering all other children from wormholes had very dramatic and depressing life stories, Eleao is assumed to just be the same way, so is given a lot of leniency. He can have brief moments where he shows his absolute disdain for humans... but they pass by fast, and he's good at gaslighting people into thinking they were imagining things.
He spends most of his time with Hau (being close to him already) and Lillie (wanting Nebby). Shizuki and Kahina both try to bond with him, but he gets annoyed by their genki attitudes fast, and wants to avoid showing that. Orane and him absolutely do not get along, with Orane probably unconsciously sensing something off about Eleao from the beginning. Being raised by a villain makes you attuned to sensing others.
Kai is a kid that lives in Hala's community and is close with Lanakila (Hau). He mysteriously showed up one day, during or after a meteor shower, and his origins are unknown.Due to these circumstances, whatever group of elders that found him have their suspicions about him... legends about aliens like Necrozma already exist, after all.
While his dark skin and white tats make him look native to outsiders, he stands out within that community. As he's mostly blind in the sunlight, he prefers to be out at night. Within his community he's already known as a troublemaker, but is even worse with foreigners, being known as a hectic klepto as well. He's strange in that he doesn't like people knowing he's blind, but likes to pretend to be mute / like he doesn't understand any English.
His love of causing trouble gets him involved in everyone's drama, and he's basically a headache for everyone around him.