This is going to seem odd, but hear me out. Were Claude and Sebastian childhood friends? It’s not much, but I have some evidence to back this up.
Exhibit A) Claude recognized Sebastian in episode 1 season 2 of Black Butler when, as far as the audience knows, the two have not met before.
In that episode, all that was visible of Sebastian before he was revealed was his mouth, and that was only in the right light. Despite this, Claude called Sebastian out without hesitation or shock. Also, to further this point, Sebastian deliberately wore a large coat and hat to hide his identity, however, his mannerisms and/or voice gave it away.
Exhibit B) Claude knew exactly who he wanted to frame for Alois’ target and was up to date with Sebastian’s life.
In episode 8 season 2 of Black Butler, it is found out through flashback that Claude told Alois that Sebastian made the contract with Luka and killed the boy in the end, and currently was in a contract with Ciel. However, in episode 11, Claude admits that he knowingly lied and knew from the beginning that Hannah was the one who made a contract with Luka instead of Sebastian.
Exhibit C) Sebastian knew where to go when Ciel’s soul was stolen.
In several flashbacks through season 2, it was explained that Sebastian was close to collecting Ciel’s soul as payment but was interrupted. Unlike the audience, he didn’t see the small spider at the location, but he still knew Claude was the culprit. He then traveled to the Trancy manor as soon as he was able.
Exhibit D) Even in death, Sebastian and Claude showed one another respect and, dare a say, kindness.
In the grand finale, episode 12, Sebastian and Claude spent the whole episode continuing a streak running for several episodes of talking smack to one another but adding in a last minute action scene that the previous episodes lacked. However, after the final fight is said and done, as Claude made his final breaths, Sebastian showed him genuine respect and they had a very civil conversation. Claude even asked Sebastian to get his glasses from his coat pocket and help put them on his face, to which Sebastian does without hesitation.
These points only prove that these two demons know each other, have history, and for some reason, hate one another. Here’s where we enter fan fiction territory.
What I firmly believe is that Sebastian and Claude were childhood friends, possibly for a very long time. Then, something very big and horrible happened between them before the events of Black Butler and they resented one another ever since, explaining why Sebastian would never have even mentioned Claude before. I’m not sure which one of the two was the one who started the feud, or even if either of them were responsible. However, what I do know, from personal experience, is that those who don’t know or trust one another well don’t hold grudges against one another as large as shown all through season 2 of Black Butler between Sebastian and Claude. A grudge or rivalry like theirs is more similar to best friends made worst.
I’ve already posted a detailed description of how I think this happened. Check out my opinion on their childhood if you’re interested. If you wanna watch season 2 of Black Butler again, to see if I might’ve overlooked something, snatch a DVD for yourself on Amazon.