Remus Lupin has had many moments of regret through the years.

He regrets letting James trust Sirius, just as he regrets forgiving Sirius for making him attack Snape that night in 6th year. Sometimes he stares out the window and wishes that he had been the one to hunt down Black, if only so that he could have gotten a good shot at him before joining Lily and James.

But standing in his classroom watching the walking mirror image of his dead friend, Remus Lupin doesn't think he regrets anything more than the fact that this boy – this thirteen year old boy, who should be worrying about girls and homework instead of a mass murderer who is determined to kill him – can't even fuel a Patronus charm because he can't think of a single truly happy memory.