Grimmjow wants a tough s/o, one who isn’t sensitive and could handle his brash personality. He’s likely to be a douche, even to his s/o at times, so they can’t whine and complain about it. It’s okay if they’re a little feisty though, he likes that. But at the same time, they can’t be obnoxious about it. He’d also like someone fearless and strong. Grimmjow has no problem sticking up for them if it comes down to it, but he lovesss a s/o with a headstrong personality who’s not afraid to give someone a piece of their mind

Szayel really needs a submissive s/o, maybe an introvert who honestly doesn’t speak much. They don’t have to be clever or intelligent, he might even enjoy it if they’re a bit curious and ignorant on certain subjects. That way he can educate them whenever possible

Aizen would probably need someone who could adhere to his cruel and antagonistic nature. If he had a s/o who reprimanded him for it, or found that side of him unpleasant, it would just be annoying. So he would like someone who wasn’t so morally uplifted that they could settle with his sometimes malicious endeavors. Someone intelligent would also be an advantage. He really couldn’t handle a ditz

Gin, despite his sly personality, would want an honest and loyal s/o. He’s all fun and games for the most part, but when it comes to his s/o, he wants to be able to drop his facade and just enjoy their genuine company. However, a charismatic s/o with a sense of humor is more than welcome