Lucanis has been taught to accept death since he was a child. He's trained in it. He's used to it. But that doesn't mean what family he has left isn't important to him. Rather it's what matters to him most.
Then he gets imprisoned and tortured. He watches people die, week after week for a year. He's made to think he's going to die at any moment. Made to think that his family is dead. But he doesn't know.
Then you come along, set him free, return him home to find only one family member left, the other probably dead after all. But he still doesn't know.
Then she's alive, imprisoned by the same person that imprisoned him, the other living relative. But he can't kill him. He won't be the reason to lose what he has left, and there's so little left.
But then there's you, who's shown him compassion he's never known, that there's something more than just family/contracts/enemies. That it's safe to be vulnerable around someone.
But he's scared to get close. Scared he'll hurt you. Scared he'll lose you, to himself, to a god, to something he can't kill.
Then his walls start to break. He doesn't know what he'd do without you. And then you disappear. You might be dead. A day, a week, a month goes by. You're still not back. You're probably dead. He thinks he'll never see you again.
It's weeks of not knowing, and he's wasted all this time fearing he'd only take time away from you, time he could have given you instead of pushing you away, and he's lost you anyway. You could have both been happier, if he wasn't so broken.
Then you're back, and nothing else matters, there's no holding back. He'll do anything to protect you, you are his sole purpose now. This he knows, more than anything.
Then the battle is over, the gods are dead, and so are you in his arms. He couldn't protect you.
He knows death. He knows you're dead.
But only now after decades without, does he remember what it's like to grieve.
