I accidentally killed Siegward of Catarina. Is there anything I can do?

I interacted with this character for the first time, and he offered to help me kill the enemy next to the tower. However, he was killed after the fight ended. Is there any solution? And if there isn't, does his death significantly affect the game? :( Will Greirat die the second time I send him to pillage if Siegward is dead? I think I read that he was necessary to keep Greirat alive, but I'm not sure. Help me please :(