It was clear that the two would become at least friends when they started to talk, but neither knew till later how their relationship would go.
Kanaya found Rose very interesting, but never really knew how to talk to her without feeling embarrassed about what she might say. After some thought, she decided to talk to Dave first, learn about what Rose likes and doesn't like, and the right away to approach her.
When Kanaya finally got the courage to start talking with Rose, she felt much better about it. They talked and talked, quickly getting to the conversation. 8thWhen Rose went Grimdark, Kanaya was worried greatly for the human, wondering why she couldn't see her.
After all that though, Kanaya knew that she shouldn't bother Rose anymore. They had to do other things, which, at the current moment, where important to them.
When everyone was at the Green Sun, Kanaya waited a bit to go say hello to Rose. She wasn't sure as to how to explain her that she is a Rainbow Drinker, because Kanaya wasn't originally a Rainbow Drinker when she met Rose. It was Rose in fact that said the first actually verbal hello between the two.
During the long 3 years, Rose and Kanaya grew very close. At first it was mere dating, but as time drew on, they seen went nearly everywhere together. They kind of forgot about everyone else, too busy with each other to help Terezi.
The only problem was though was that Rose was drinking. Rose wasn't at her best, but Kanaya stayed with her. When Rose had one to many, Kanaya was there to help her back to her room. Many times she had to stay with Rose in her room to make sure she didn't puke everything that she drank, which happened a lot.
Kanaya didn't quite know why Rose was drinking, but after some small talks with Dave, she had an assumption. The drinking reminded Rose of her mom, and Kanaya respected that. Even though it was harmful for her to drink so much, Kanaya let her drink. Wasn't the best decision, but they are only kids, and kids make mistakes.
They made it through though, all because of the fact that Kanaya stayed with Rose through all of it. She didn't even think once of leaving Rose's side, as if she was paying her back for standing by Kanaya when she was down in the dumps. It was sweet to look at for Karkat and Dave to look at, and they can see that the two will be great together for a long time.