[nit3mar3]

Spoiler

there wasn´t much of a fight between tousen and kensei. that´s what backstabbing is all about, that there is no real fight.
and, aizen more or less raised gin to be the man he is now, he took him in as a prodigy and his protegé. tousen kaname was very much the only blind shinigami, so he had to pull him to his side. tousen´s calm demeanor makes him an excellent subordinate for anybody. no denying their powers, they wouldn´t have been captains if they were not extremely powerful, but don´t count out their allegiance to aizen when wondering about why he allows them to stay by his side.

btw...kenpachi´s brute strenght is without question, tb, there´s a good reason why he´s called kenpachi, but what else does he have...