I don't really think anybody knows who he is. As far as I know, he has no name, and isn't a super important character, unlike Red, who you meet later on.
If you explore the level with this mysterious being in it, you will find that he is likely the scientist who wrote those diary entries, concerning the digestion of spiders, and escaping the mine for freedom. That is all I really know...
Oh, and he cut out an 'external organ' (I think it's his tongue) and placed it on a shelf, and he has been eating spiders for a period of months...making him a little loony.