This is my theory on how the servants would find out Sebastian was a demon, if they ever did. it can be read as part of the lead up to the events of "a battle for a soul" but isn't directly related. A guest reviewer asked how they found out so this is my answer.

takes place after Elizabeth and Ciel are married.

The servants sat around the table after Sebastian had told them there daily jobs before he went to wake the Master and Lady Elizabeth. He hadn't been gone five minutes when Finny asked a question.

'So, does anyone know what Mister Sebastian is?'

The other servants stared at him with their mouths open, so he decided to explain.

'You guys probably thought it was common knowledge, but I am not bright so I need to ask. So, what is Mister Sebastian?'

'What the Hell are you talking about, Finny? Sebastian is weird as hell and a monster at times but the guys still human. Why are you asking about this only after almost five years?' Bard demanded.

'Well; May-Rin has really good eyes that make her a good shot, and you were trained in the army so you know about explosives. Tanaka was raised to be the Butler of the Phantomhives and worked under the previous Guard Dog, and Snake can talk to Snakes and control them. Then I am a failed human experiment. So my question is; what is Sebastian?

'I don't remember him ever saying anything about his past, and there are all these little things. He's stronger than me and I was modified, I once gave him bulbs and he used fully bloomed flowers, he does the impossible on a regular basis. He came back to life after being whacked on the head and stabbed thorough the chest. I know of times when the only explanation was that he was in two places at once.

'My point is, we are all weird and it's questionable whether we're human, and he can do stuff that even we can't. I never really bothered to think about it, but it's obvious when you do, so since you're brighter than me I thought you could tell me?'

The servants looked at each other and realised that Finny was a lot smarter than they had thought, because it hadn't occurred to them to even question the things that he was mentioning. The only one that looked like he might know more than the rest was Tanaka; Snake hadn't been around for a lot of the events that Finny was describing.

Finally Tanaka decided to tell them his theory.

'Sebastian loves to joke that he is "one hell of a butler" and some of the Japanese phrases he uses can also mean "I am a demon". Maybe there was more truth to these statements than we originally thought?'

After serval minutes of discussion the five of them were decided, Sebastian must have been secretly confessing the truth in his own subtle way.

'So, what now?' Asked May-Rin; feeling slightly sick at the idea that she had been crushing on a being from hell. 'What do we do now that we know that Sebastian is a demon?'

'Do we need to do anything? It's been like that the entire time we've been here, why should the fact that we know make a difference?' Finny said, sounding unusually serious.

'I never thought I would say this, but Finny is right,' Bard said. 'It's been this way always; and demon or not we own our lives to those two. If Sebastian is a demon, if the Master sold his soul to him, none of that changes the fact that without those to we would all be dead or worst. They gave us a roof, they gave us food, and they even gave us education and jobs.

'So we do nothing, this changes nothing. If Sebastian decides to take the Master to hell, then we follow them there, it's as simple as that.' The others nodded in agreement, realising just how right those words were.

It was then that the man in question burst through the kitchen doors.

'I gave you five your jobs hours ago, why on earth are you still sitting around talking? Get to it!'

And if the servants moved a little faster with the knowledge that the Devil horns on an angry Sebastian weren't entirely imagined, well, no one commented on it.