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"I'll be right behind you." Aether promises, nudging Lumine forward. "Go on, Sister. I'll follow in a heartbeat."
Lumine frowns, one hand outstretched, her fingers dissolving into stardust. "I don't see why we cannot leave together, Brother."
"We will be. It is just a matter of shifting; I am tired, and it will take a bit of time. We will always be together, so do not worry." Aether smiles, crinkling the skin at the edges of his mouth and eyes. "I need to look strong in front of my little sister, no reason for you to see your Big Brother tired."
Lumine huffs, but it is not the first time something like this has happened; sometimes, she was the one who was weary, or loved the world they were about to leave too much. So she lets herself go; dissipates into motes of light that gather together in the shape of a star, and for a moment she cannot feel the warm glow of her brother-
Then she sees Aether smile at her through the light, and he, too, morphs himself into a piece of the sun. She draws closer, affectionate, and Aether pulses back in fondness before they're streaking through the sky and into space, reverse shooting stars as they make for the next planet.
Lumine feels content, dreamy in her light form, and she spins in a ribbon of light, joyful laughter lost in a place with no sound.
The thing you must understand is that for millennia, Aether and Lumine were names linked together. Wherever they settle, it only takes a while before the world’s occupants realise that where one is, the other is typically close by. There is no Aether without Lumine, no Lumine without Aether, and every world they traverse understand this, one way or another.
There are foolish humans who wish to capture them, arrogant gods wanting to claim one twin or even both. They are killed for their sacrilege, twin swords flashing as wings of gold slice through those they have judged unworthy. And that is alright. For Aether, Lumine would give him anything. For Lumine, Aether would bring the galaxy to its knees.
(They are the Travelers, the Golden-haired Twins, hand-in-hand, side-by-side, exploring planets and watching stars die.
They were always together, until they entered Teyvat and found themselves in a world filled with peace and war.)
It goes like this; Aether and Lumine streak across Teyvat's skies, observe seemingly calm lands and wanton destruction, the presence of gods strong. They decide to leave after a short break in their personal forms, and fly with shards of wings far over the countries down below. Lumine watches Aether's braid flow behind him, flapping in the wind, and thinks that she'll have to rebraid it.
They find a strange platform floating in the sky and alight on it, curiosity getting the better of them with the lack of any presence. They intend to rest here briefly and explore before they finally leave this world behind, until a god steps out from red-tinged shadows and proclaims them to be intruders.
Instantly, their swords and wings are back out, glinting in the light as they take off, moving in tandem, millennia of fighting together in their bones. Strike down the god, and move on. Their opponent is unknown; caution is advised.
Lumine ducks and dodges the strange red-black cubes the Unknown God commands, weaving through the sky and seeing Aether do the same nearby. They twirl in the air together, shooting for the Unknown God, swords ready to strike. And then-
Teyvat's god runs her power through Lumine's sword, and she jerks back the moment she senses the taint, before realising she has left her brother's side. In the very same breath, the Unknown God breaks Aether down into bits, condensed into a cube that glows in a pale mockery of her brother's light.
A single heartbeat is spared to understand this, and Lumine rages. She lunges for the Unknown God, fury in her eyes and power in her veins; how dare she take her brother away, how dare she separate them. She will rip this world apart with nothing but raw power and make the foolish god return her brother-
And red cubes encase Lumine in her moment of vulnerability, in that window between her mortal form and her true one, a moment before a supernova was unleashed.
Lumine shouts, a desperate last cry for her brother as the Unknown God turns away, taking Aether with her. She feels, distantly, a large presence behind her, something cradling her wings and crushing. Her scream is muffled in the darkness as her wings - different from her brother by colour alone, another thing in a long line of similarities and differences between them - are mangled, torn from her like paper.
They are ripped from her back, pain blossoming at the connection points, her cries of agony lost to the endless void even as her mind goes foggy with exhaustion. The Unknown God took her twin away, and Lumine did not even have time to mourn the loss before her wings were taken too.
Her last thought before the darkness claims her is to destroy the damned god, and take back what they stole.
(Aether falls from the sky, and his last thought is, I must find my sister.)
Lumine wakes up in a foreign world, alone, without Aether by her side, and shatters.
"You said you would be right behind me." Lumine mutters into her hands, pressing them to her face. It is a stupid thing to focus on, but her brother promised and yet he isn't here.
"Please come back." Lumine keens, feels the prickling, painful absence of her wings on her back, the numbness tied to loss of much of her power. Lumine swipes angrily at her tears, fixes an image of her brother in her mind's eye, crafts a goal. Then she stands up, and starts setting about finding resources.
(She decided, at first, that she would rip the Unknown God's core from her, take back her brother, and use their powers to reduce the world to cinders. Instead, Lumine finds herself growing fond, and decides the world can stay, when she truly begins her destruction.)