User blog comment:B214/Light Novel Volume 16 spoilers/@comment-74.110.105.73-20131018150452/@comment-5185939-20131019174628

Death Flag means he does something that authors often use to foreshadow a character's death.

Think of it as a bad omen for the character.

Even if he doesn't die, he'll likely have at least a near death experience which may take him out of the story for a while.