Both Derek and I will go to the dance etc...
Remove the other character and try to write the sentence as if it was just yourself doing the action: "Myself will go" or "Me will go" don't sound right, but "I will go" is correct.
Answer:
It is a pronoun
Explanation:
A pronoun is taking the place of a noun. Since you could replace a noun with it, like instead of "the book didnt move" it would be "it didnt move". While this is much less specific, it still counts as a pronoun.
