Levi was once a human already promised the grace of an angel when he dies. But the world was cruel and corrupt and him being hit by reality and demons turned him away from the heavenly call at death. Humans gifted with grace are very powerful angels, but they can also turn into powerful demons if led astray.
Eren is a powerful priestess gifted with grace since birth- as she was favored by the heavens and fate. She can see spiritual beings, and command them at will, and that makes her a powerful ally or enemy. Which is why Levi was back o earth, the devil at her shoulder, tasked with the important duty of turning her to the dark side.
She’s still the same, though, and when she finds Armin and Mikasa again nothing’s really changed between them - she still gets into fights and trouble and the three of them are inseparable as always.
But once she hits puberty she sort of realises that if she ever finds Corporal Levi again, would he still want her? Even if they were together in another lifetime, Eren can’t help but worry that whatever quirk of biology that left her body female instead of male would be enough to have the Corporal turn away from her in disgust.
But she does find her Corporal, and he’s as pale and beautiful as she remembered - a bit taller this time (it must be access to better nutrition during his formative years, she thinks), and she’s a little shorter, so for once she’s not looking down at him, but eye to eye.
Her elation at finding him is soon replaced by apprehension when he finally notices the glaring differences of this lifetime. She fidgets, and mutters that she understands that her body is different now, and she won’t bother him for much longer. But before she can leave his hands (still cool and calloused and perfect) wrap around her wrist and pull her back, and his voice is tinged with exasperation when he scolds her for being melodramatic, of course he still wants her, he fucking sacrificed his life for you, you shitty titan-brat, do you think boobs and a lack of a dick will change anything?
Eren flinches at his harsh tones before her brain can process what he’s actually saying, and by then Levi’s face has softened as he pulls her close and says exasperatedly, “You’re still Eren, right? The form your body takes is incidental.”
Eren smiles, giddy with relief and replies, “Shit Corporal, I’m sorry.”
She ducks her head down and murmurs softly, “I still love you too."