Zombie-Loan, Vol. 1

Michiru Kita's a weak-spirited girl who has a hard time saying what's
on her mind. One day, she notices mysterious, ring-like markings around
the necks of two of her classmates, Chika Akatsuki and Shito Tachibana
- two boys who miraculously survived a horrible accident six months
ago. Michiru, possessing the rare ability to see these rings, knows
that they warn of impending death. Thinking that, perhaps, she can do
something to save her classmates' lives, she approaches them...but it
seems the boys have already made a different kind of deal and garnered
themselves a heavy debt.