I don't care whether if they get an increase of dmg when fighting demons,THEY WILL STILL GET OWN. You don't see any SAM soloing or pting cause they already know what the deal is. SE need to use their head and come with a trait cause this suck. :D
Heh, for you, or anyone else who needs to know, you don't encounter very many real demons during the course of your early adventures. Later on however, as you venture into Xarcabard and Castle Zvahl, you're going to glad to have a Samurai along.
Demons are basically another class of beastmen. They are predominant in Xarcabard, and Castle Zvahl, and probably can be found in some Promathia (Are fomors considered demons? Probably not) areas I haven't been.
The Floating Eye class also constitutes a demon. These are spattered in the deeper areas of Ranguemont Pass, Beaucidine Glacier, and of course, Xarcabard, Zvahl, and more areas I haven't been.
Contrary to some belief, the "evil weapon" class is not considered demons. Or if they are, demon killer and warding circle are not going to have an effect.
As of the december patch, all "killer" and "circle" effects now add an automatic paralyze-like effect to the referred-to mob type. It gives a "[mob] is intimidated by [player]'s presence" message. Note that dragoon-type mobs can use this against dragoons' wyverns also. Killers and Circles and Paralyze can all be stacked. Circles last 1 minute and can be recast every 10 minutes. This information is from Squaresoft's patch notes (except the stacking part, that's from my experimentation).