the_debt_of_time

"Let us speculate then, Remus. No one knows Sirius better than you."
He had known Sirius Black, or at least he thought he had. "Don't assume anything
about me either, Albus."
"What do you think Sirius Black would be willing to do to get to Harry?"
Remus shook his head remembering the way that Sirius used to hold his godson
after he was born. Like a treasure. Like his own son. "Anything."
"And should he find Harry?"
"I... I don't know." Remus sighed. "Albus, I'm not... I can't be the person to
deal with this. I'm too close. I don't know what you're trying to make me do here."
Dumbledore reached into his desk and removed from it an aged piece of
parchment. He slowly unfolded it and lay it open to Remus's eyes. "I was once assured, a
long time ago, by a very wise witch that Remus Lupin, above all others, could be trusted
and depended on."
Upon seeing the letter—Mia's letter from him from the future—Remus reached for
it, only to have it pulled away from him at the last moment. "Where did you get that?"
"After Miss Potter left us, I managed to acquire it. I could not, in good conscious,
let something so dangerous fall into the wrong hands." The implication of those hands
belonging to Sirius was left silently hanging in the air. "I'm well aware of your past
relationship with Mia Potter and, I suppose, Hermione Granger's future relationship with
you―"
Remus grimaced. "Please stop saying 'relationship.'"
"Can the word of Mia Potter be trusted?"
Sighing, Remus answered, "Above all others."
Dumbledore scooted the letter across the desk, nodding with approval as Remus
snatched it up into his hands with desperation, like a child would a beloved stuffed animal
or security blanket.
"She put a great deal of faith in you, Remus. I feel Harry puts a great deal of faith
in her, and right now both need to be protected. If an unstable Sirius Black somehow enters
Hogwarts to find Harry, what do you think the man capable of, should he stumble upon
Hermione, a girl who is very nearly identical to the love of his life?"
Remus felt sick at the thought. "I'll stay. I'll stay and protect them both. It's my
job. My job to keep them safe."