It seems that "Here" is only an adverb. That means it has no other part of speech except an adverb.
here is/are something (also here it is/here they are)
a) used when you are giving something to someone, or showing something to them (source)
Ok, let see this conversation
A: could you give me the pen?
B: Here is your pen / Here it is
Can B reply "Here is it"?
It sounds pretty strange when saying "Here is it".