Many non-trinitarians believe that it's an agent of God - Muslims teach that it's an angel - or it may be impersonal aspect of God - this is the traditional Jewish understanding, if I've read them correctly. The word in Hebrew means breath or wind more than it does "ghost," like an invisible person. It is something poured (Isaiah 44:3, Joel 2:28) and you usually don't pour people.
Also, when we write about the Holy Spirit, we usually write "it" rather than "he." We don't do that in English with people. With this kind of language, I can understand why people think it's impersonal. Why not call the Holy Spirit a "he," like the other two parts of the Trinity? I don't know anybody, personally, who does that.
We can believe that it's God, of course, but not because the only other choice is Satan.