This answer is coming from somebody who has only seen Black Butler animated and is completely ignorant of the manga, so this answer may not be completely accurate.
Does Sebastian care for Ciel? Well, he cares for Ciel's life, he cares for Ciel's resolve, and he cares for Ciel becoming a tasty meal for him. Does he see Ciel as a friend? Probably not.
In Book of the Atlantic, Madam Red tells Sebastian that he is practically like family to Ciel, in which Sebastian responds by saying "I would never presume to think of myself as family, that would be inappropriate. No, I am merely one hell of a butler." I assume that the same would go for friendship. This is further backed up in episode 15 of the first season, where Agni refers to Sebastian as a friend, and Sebastian is caught off guard, explaining that nobody had ever referred to him as such. I doubt that Sebastian sees any friendship between him and Ciel, at least nothing beyond master and servant or contractor and contractee.
You mentioned that Sebastian is always loyal to Ciel, and this is because it is a part of their contract. Sebastian is ordered to protect and never betray Ciel, along with never lying to him, and following his orders to their completion.
When Ciel and Sebastian's past together is revealed in Book of the Atlantic, you can see that Sebastian is initially very annoyed with Ciel, which makes sense considering that he is a demon having to play the role of a butler for a bratty little boy. But over time, I believe Sebastian grows to tolerate Ciel in the same way that Ciel tolerates him. They'd been together for three years and spent a lot of time training each other to be the perfect earl and butler. So while they may not be friends, they've been together long enough to not really mind each other.
I think it's also very important to note that while Sebastian only wants Ciel, Ciel needs Sebastian. I believe it is a lot more likely for Ciel to care for Sebastian than it is for Sebastian to care for Ciel considering that Sebastian has been Ciel's primary caretaker since his parents' death. However, while more likely, it isn't a very high chance that he actually does. Had Ciel actually gained those feelings towards Sebastian, I would imagine that Sebastian would have considered him too weak to complete his goal, and therefore eat his soul right then and there. I mean, what kind of earl would desire a friendship with a mere servant?
This is only my take on things, so it may not be entirely right. I believe the relationship between Ciel and Sebastian is very much up for personal interpretation, anyway!
