The transition happens too fast. One moment Shikamaru is dying, his heart destroyed, pain beyond imagining sparking along his neurons, blood clogging his throat in his death throes. The next, he wakes up, gasping, impossibly, his sister’s crying face the first thing he sees out of the void.
The next, her eyes go dark, expression flat. Her grief and relief erased, replaced by apathy.
Shikako dies instead of him, and Shikabane takes her place.
Shikabane plays the part, dutiful Konoha shinobi, dutiful Nara daughter, dutiful twin sister. It is a lie. Shikamaru knows this, but he still plays along because surely it’s better to have this fake than nothing at all?
But even in her new existence, the creature that was once his sister puts him first.
“You should say goodbye,” says Shikabane, tugging at his hand. His shadow hand, specifically. There’s some sensation in it, enough to tell there is contact, but not much in the way of detail. It can’t differentiate sensations: Shikamaru wouldn’t know if Shikabane’s hand is soft and warm like his sister’s would be, or if it’s as cold and hard as stone. As a demon’s lack of a heart.
“I,” Shikamaru hesitates. The face staring impassively back at him is still his sister’s. “I don’t think I can.”
It’s not as if Shikamaru wants to die. He very much enjoys living, thanks, he’s not that lazy.
He doesn’t want to die. He just doesn’t want his sister to hurt herself for him even more than he wants not to die.
But he cannot change the past.
He’s grateful to still be alive, he just wishes it hadn’t had such a high cost.
He’ll tell his sister thank you only when he manages to get her back.