He wasn't really like her. Not in a very obvious sense. Disregarding all their differences personality wise, he just simply at the end of the day wasn't human. His skin was plastic, white with a streak or two of grey in places. He was strong, stronger than any human could ever hope to be. Yet she seemed to best him at all feats of athleticism.
After the time it took for her to forgive him, and for him to forgive himself, she said she didn't mind how he was. She didn't mind that he was an android and that he didn't really look or act human. He minded however. He started out hating humans for everything they had done to him. They had created him simply to be a failure and be an idiot.
When he came to the outside world, met with other humans that didn't seem to mind what he was, who didn't treat him differently or scorn him for doing something wrong but instead tried (usually unsuccessfully) to teach him how to do something right, he began to accept humanity and humans in a much better light. This isn't to say they accepted him entirely, they would get tired of his questions and failures or the choppy way in which he moved would put them on edge. There were a few who found him to be quite annoying in fact. But in comparison to the labs, this was far superior.
He did have issues fitting in at first. While the humans were used to robots, an android of his intricacy and detail was essentially unknown. While he was working for Chell's trust and living on the streets he made himself rather well known around the little farming community. He would talk to people that lived there, at first they were startles by him of course but news of this strange charming little android spread fast, and people were seemingly happy enough to have a small chat with him about almost anything.
Of course he almost always asked about Chell. Where she lived, what she liked, if she had mentioned him, if she ever talked about aperture, if she ever actually talked at all, and most importantly if she would ever forgive him.
Of course eventually after enough groveling and tears she did and he moved in with her.
The more he was around these people the more he wanted to be like them. He wore human clothes and hats and helped Chell with her farm work, going into town and helping her sell it as well. But there was something…missing almost. He had become so ingrained with these people and they had become a part of his life just as he had become a part of theirs. Yet he was not like them.
When the realization that he was not human, nor would he ever be human actually occurred to him he simply felt lonely. For the first time in his long robotic life, he wished that he was in fact a human. He told Chell about this later and she (as usual) didn't say much, just looked at him with his white plastic skin and gave him a small sad smile. He buried his face into the crook of her neck and wrapped his arms around her. He was careful, always careful, that he didn't squeeze or push her. He suspected that she didn't know the extent of this. He was just simply tired of it; tired of everything associated with being like this.
That night, long after chell had gone to bed he sat up looking at himself in the mirror. He ran his white fingers over his white face, looking back at black eyes with blue light irises. No human looked like this. He sat there for a long time. Getting mad about his features and that he couldn't do much of anything about them.
Then, as he finally got up to leave he noticed her small assortment of makeup. Chell wasn't the sort of woman to have a lot of makeup, she had what she needed if she was to go somewhere formal, but other than that she rarely ever used it at all. Wheatley smeared the brown paste onto his fingers. Chell was dark skinned. Much darker than he suspected he'd be if he were actually a human.
Regardless he smeared the paste across his cheek, the darkness of the makeup covering the pure white of the plastic in very patchy uneven blotches. He tried his best to even it, rubbing it back and forth with his fingers to smooth it out across his cheek. It didn't really work.
He looked as it he had smacked the side of his face into mud.
A small noise came from the door way and he was startled to see Chell leaning against the frame. Immediately he stood up and started apologizing and backing up, hiding what he had done to his face with his hand. He hands slowly reached out to him, claiming his wrists and pulling them down to his waist to see what she had watched him do to himself.
She gave him a small sad smile.
He refused to meet her eyes as she sat him down in front of the mirror again. She wiped it off slowly with a towel, the makeup came off easily, not sticking to his plastic features. She brought his head to the crook of her neck, and wrapped her arms around him, a similar sight to what they had done earlier.
He let out a synthetic sob and she stroked his back. For the first time she gave him a kiss to the top of his head, smelling the plastic white hair and she burrowed her face into him.
She liked him as he was because no, he wasn't human. He didn't judge her like humans judged. He only knew the humans of the lab, he didn't know real people. He didn't know they talked about him behind his back, and only put up with him from the front.
She didn't want him to know how awful humans really were. She didn't want him to succumb to the things humans do to each other simply because someone might be different. She rocked him back and forth, sitting there in front of the mirror. He wouldn't understand. She suspected he never would.
She could only encourage him to be what he is, because some things just can't change. He was obviously not very happy with that as an answer but he had no other choice. She took his hand and led him to her bed, where she gave him another kiss on the top of his head.