It's amazing how their friendship evolved from the mere need to help each other out. Since then they have always been together facing the most deadly challenges and it has become a lifelong friendship.
Actually they are just master and servant. But those two have established a very close relationship. They are dependant on each other, not just because of their contract. Ciel may be cold-hearted and cynical at times, and Sebastian never ceases to tease his master. But it's not hard to see that they have become attached to each other. :)
I seems that I am somehow attracted to love-hate relationships, I don't know why. It just makes everything so much more interesting. A quote says is best: "You can't kill me without becoming like me and I can't kill you without losing the only person who can keep up with me."
The rivalery of those two is so entertaining. And as one can see often enough under different circumstances they could actually be friends.
The thin line between that makes it such an interesting relationship.
Alright. I just wrote. And then I read it. It's cheesy as hell and weirdly written but meh. I'll go revise this list some other time. Maybe I'll come across more relationships that really touch me, then I will add'em.