I'm alright with you making him the 'alucard' of butlers - at least in terms of personality. Please do not attempt to fill him to the brim with extravagant feats' of power, impossible to stand against.
Yes, our butlers are demons, but this is also a group RP which means everyone needs to be on an equal playing field, lmao; and for anyone who has seem/read Hellsing, Alucard is practically immortal-- no strike that, he is immortal.
You're more than welcome to impliment how his Master/Mistress has placed seals on him in order to contain his power. Feel free to discuss that with your partner once you've acquired one - perhaps even go about working that into the reason your butler serves said noble.
As for the sun-glasses idea... I'm not entirely opposed to it, seeing as it may serve as some hilariously rich comic relief, so go for it :)