Identity and the birth of sky children
Every now and then when Orbit sheds a piece of its light, that light falls upon the earth. Sometimes, this light, yearning for an identity, takes "inspiration" from the environment to take on a different form. In the case of light creatures, their forms were inspired by the creatures of flesh before them — creatures like crabs and fish. From them, they inherited the ability to sexually reproduce, allowing them to evolve more quickly over time as opposed to waiting every few years for a new light creature to be born to introduce genetic variance. This is how the Ancestors came to be, primarily evolving themselves into existence.
In the case of sky kids, who currently only come into being by falling from the sky, their forms take after the Ancestors/Spirits. The two share many similarities, though one notable exception is that sky kids' "inner" light is on the outside of their body, similar to other light creatures, as opposed to being on the inside like the Ancestors. Compared to other kinds of creatures, sky kids are rarely born, as the form and intelligence of the Ancestors/Spirits is difficult to replicate compared to other creatures.
When a sky kid is successfully born, they are also born clothed, mimicking the Ancestors who also mostly had clothes on. The specifics of their appearance is influenced by the realm they were born in.
ID: A diagram showing the possible things that can happen when light from orbit, lacking an identity, falls to the earth. The light could not take a different form, eventually dissipating and becoming a part of the earth's ambient light (and potentially becoming a roaming light). The light could become a light creature, being able to sexually reproduce once reaching maturity. Lastly, the light could become a sky child, unable to reproduce even when mature since they take after the Spirits (who also can't reproduce by virtue of being Dead).
More on individual appearance
While the bodies of sky kids [each] have the similar features, what clothes they are born with depends on what the Spirits wore in the realm they were born in. Being a child of the Wasteland, Mufo's initial appearance was influenced by military-type Spirits.
ID: A drawing of the Fainting Warrior and the Lookout Scout, "rather typical Wasteland denizens". Between them is a block of text reading: "Triangular chest pieces separate members of the military from citizens. The Warrior's chest piece is made of metal, while the Scout's chest piece is made of a rigid cloth. Metal face crests are also common, but only required for combative personnel like warriors."
ID: A drawing of Mufotsuki after falling from the sky and fully forming, being born with the physical appearance and maturity of a 9 year old. Some parts of her appearance resemble that of a typical 'moth', however she has a rudimentary face crest and triangular chest piece, mimicking Spirits like the Fainting Warrior and Lookout Scout.
ID: A drawing of young Mufotsuki and the Lookout Scout standing next to each other, demonstrating how fledgling light creatures' abilities to replicate other creature is good, but not perfect. They are physically similar, though Mufo's clothes are missing details compared to the Scout's.