Disclaimer: I do not own Naruto. I just run around and play in the realm.
Before Uzumaki Naruto
Before Uzumaki Naruto, all Sai saw were blank mannequins of people without names. Lumps of flesh and bone attached to faces he was not able to read. Not that it was necessary. People were tools, targets, or leaders. Over time, that had become clearer, then cemented after the death of his surrogate brother.
He thought (rather, pushed the possibility of musing about it so deep down that it'd never resurface) he would find anyone like his brother again. After Shin died, no one else in Root mattered to him. He followed Danzo's orders without question, as he'd been taught. It was all he had ever known. Root was his identity, to be a cog in the wheel - but he painted.
He painted everything. Some were sketches, most were what he had observed and found interesting that day and captured it to paper. The paintings of his brother were most important.
The picture book held his memories - even if they erased what was in his mind, he'd have tangible proof of his tie to someone, the link to Shin that had faded but did not disappear. Unspoken words Sai might have said to Shin rattled around in their cage, most without his knowledge.
Sai was almost always alone in Root. His life consisted of brief conversations with Danzo, missions, training, and sleep. If he did work with others, it was without defining names, few words, and no passion.
Naruto was different. Danzo failed to take into account how odd it was for Sai to be around the members of Team 7. Sai's control on his emotions was ever-present; Naruto lacked restraint in general. Sakura was a strange force of nature herself.
First, observing the enemy, or co-worker, was high priority. Sai tested his boundaries with Naruto and Sakura and added to his growing list of mental notes. His mental notes fell into the categories of human, personality, or environmental observations. When two or more of them blended, the result confused him.
Even so, his observation and definition of Naruto grew. Naruto said what he meant, and meant what he said. The straight-forwardness was relieving and intriguing compared to the games other people played and the lies they told.
Sai and Naruto knew loss; Naruto felt it so deeply that he could think of nothing else and needed whom he considered family around him close so he'd never be alone again. Sai wondered and maybe hoped, in the recesses of his mind, that he would be part of an important bond again one day.
Humanity began to peek through the cracks in his Root mask. He wondered when the mask would shatter.