Sirius was brave, because Remus could not be.

Remus was quick witted, for Sirius never was.

Reason and rationale came to one because it was all he ever had, all they ever had was hours alone to meticulously plan his every breath and action. If his heavy mind had not been so many years ahead of his body, he would have been slaughtered years ago.

The other had only survived from the quick emotionally immature responses to his mother swatting him with a rolling pin or ducking through the front door while the house was asleep to seek refuge at James.

Like two halves of a lopped melon one completed the other in every way a person can be completed.