Disclaimer: Are these actually necessary? Not mine.
Everyone thought he felt nothing: the statue; the silent; the child who did his bidding, whether it was to stab his own brother or to baby-sit some woman. He did it all, and without a change in expression.
He hid the irritation very well. Maybe someone very perceptive could see it; but he got it past even Aizen-sama. That feeling that blossomed in his chest when everyone murmured their assent for him to do a dirty job, because he never felt and therefore, it was easier for him.
None of it was easy for him.
But he didn't say that.
He had emotions, albeit slightly duller than those of living people. But there was no denying what he felt: annoyance, anger, pride, irritation, sadness, boredom, excitement, joy, hate… and fear.
There was an emotion he wasn't familiar with. One that only plagued him for a brief second after killing someone, or after lying blatantly to Aizen-sama and saying that yes, the woman had been behaving.
It was guilt.
But he felt it around the woman often enough. His face was blank whenever he walked into her chamber, only to guard his frustration at having to be in there. Out of everyone, the woman was the most infuriating to be around. She never spoke, although that suited him just fine. But the look in her eyes… whenever she dared to glare at him, he could see those thoughts churning in her mind, understand her very own feelings deep in her heart.
She, too, thought Ulquiorra was heartless. A beast for locking her up; a monster for showing no emotion because of it; a fiend for denying her freedom; an enemy for putting down her friends. He was everything she wasn't, and then some.
It made him angry. But more than that, he would see the dullness in her eyes grow by the day, the hopes she held the first night slowly dimming as time wore on. It gave him a twinge in his conscious. He moved on. Time wore on, as well. And each day, she looked worse. More limp, less light. Her eyes would be heavy, although she always managed a glare at him.
One night, after leaving her cell, he realized what the emotion was.
Guilt.
He felt guilty for crushing her hopes, one by one. He felt guilty for giving her no air, nothing to make her smile (now that he thought about it, he had never seen the woman smile. Why did it bother him so much?). Guilt began to swamp him slowly, creeping through his veins and clogging his heart. There was no denying that he felt responsible for her sadness, her depression. It had never occurred to him before. But there was no escape from this plaguing shame, but he knew there was nothing he could do to ease her pain.
So everyday he walked in, face masked. He felt the guilt as he ordered her to eat, felt the guilt as she stared him down from across the room, huddling in the spare moonlight filtering in through the window. He felt the guilt as he reported to Aizen-sama, struggling to hide the very blame that he was feeling.
For once—just one damn time—he wanted to see her smile. Just so he would know he wasn't the only one left with emotion.