The Most Qualified Among Them
Not surprisingly Shiro was able to figure out who Kayano was all along after observing Class E throughout the run of the assassination project.
Having realized her true identity through the subtle reaction she’d made during his first introduction to the class, he was able to decipher from his assessments that she was none other than Yukimura Aguri’s little sister, Yukimura Akari, who was able to steal their research during the confusion of the accident;
which most likely implies that she was there during the time that her sister had died.
Given her level of intelligence, plus the fact that she possesses an overwhelming tenacity to have been able to keep up this false pretense for a year, there isn’t a doubt in his mind that she is the most qualified among them to be an assassin capable of killing Korosensei.
It looks like what the class was missing to become 'true' assassins was something far more simpler than the simple motivation of money or desire to be acknowledged.
While talent and skills are things that can be acquired overtime through training and experience, pure motivation is something that you must already have in order to go farther than the rest.