Saika can be overcome by people with significant emotional trauma. Anri's own is what allows her to wield Saika as she can.

(However, -- would be too emotionally unstable to resist. He would put up a fight and might manage to regain fragments of control, but he ultimately devolves into a chaotic mess. He would eventually kill himself.) 

Luka is too emotionally weak to resist Saika. She would be driven insane. 

Edith is a strong person, always has been. However, she's begun to crumble inside in her years as a magical, so while for the most part she would have control over Saika, those instances in which she falls victim to her anger and her sadness Saika would influence her actions for the worst.

Wescott is a being that cannot love. She wouldn't be affected by Saika--she'd only sense a foreign entity attempting to enter her, alarmed because she doesn't know what it is and if it could succeed in possessing her or not. However, Wescott isn't human and Saika only loves humans, so she would withdraw from the attempt to possess her.  

Zachary would freak the hell out as Saika took hold of him. For he too is fragile and feeling, he'd easily break and become a slave to her voice.

Kyra would be able to reap back control from Saika. She might even be a capable user. However, being a child/wielder would cause her to continue keeping a wall around her heart, afraid that if she did, she might lose hold of herself.

I don't think Saika would want to take control of any other character from this verse, considering the rest are non-human.