This being appears to be made of quicksilver or mercury. His movements are graceful, if somewhat odd, as if trying his best to emulate the people he sees in his surroundings. When he is distracted, his joints tend to bend like liquid rather than hinge like bone.  His expression is also somewhat blank when left alone, though when encountering people he does his best to put on amiable expressions suitable to the situation.  

Anyone with an alchemical background would quickly recognize Slithe as an artifical creature, though it likely wouldn't take most experienced people long to guess anyway after a few minutes of observation or conversation.