Disclaimer: I don't own ML, obviously
There are three requirements to being a Cat: you must be brave, you must be clever, and you must have bad luck.
Adrien fulfills the first one, what with his always take the hit for Ladybug shtick. That one concerns Plagg. Right now, she's the important one, the one who can purify the akumas, get rid of threat, save the day. But he knows this won't always be the case. Soon Adrien will have to step up, when his Cataclysm will be the only thing standing between humanity and utter destruction. (Ironic, right?)
His Cat is clever, but he rarely uses that brain of his. Plagg can count hundreds of times where his powers could've been used differently, or when using Cataclysm was completely unnecessary, or when the answer to his beloved's identity was right in front of him. (Seriously, she sits right in front of him.) But the potential is there, Plagg is certain of that. Adrien relegates himself to the position of sidekick, but he will fulfill his duty when he has to.
But Adrien has got the bad luck part down pat. He knows that from the outside, that doesn't appear to be the case. Wealthy, good-looking (from mortal standards, apparently,) smart, and kind. However, he is the god of bad luck and destruction, and he sees how it really is. Lost his mother, is stuck with a cold bastard for a father, no real friends for several years, besides the brat with the blond hair. And the regular stuff, too, like pop quizzes after an akuma attack, accidently dropping his homework after school, a shoot right before patrol, the servants noticing conspicuous bruises...
Anyways, Adrien is unlucky. That is the most important part of being a Cat-the ability to bear that burden every day, and still smile. The power to take bad luck and turn it into something good. And Adrien has that in spades. Life has thrown much at him, but his spirit has remained as strong as iron.
So, Plagg knows Adrien is a true Black Cat. He might not be the smartest or the best fighter, but he has the capability to grow into what he was truly meant to be. Bravery, cleverness, and bad luck. He's got it all.
"Plagg, claws out!"
Master Fu certainly made the right choice, he thought, entering the ring once again.