Correct. Forget about the cats. That was my attempt at humor.
It seems to me that you are equally interested in answering
- the question: "What is the Holy Spirit? i.e. what does that concept refer to?" and
- the question: "Which Christians believe in the Holy Spirit?"
Regarding question #2, as far as I know all Christians believe in the Holy Spirit. There may be some people here in RF who call themselves Christians who do not acknowledge the Holy Spirit, but they are not traditional Christians. So, I'll let them speak for themselves.
It is also a fact that all Christians do not refer to the same thing when they say they acknowledge the Holy Spirit. Traditional Christians generally do refer to the same thing. Jehovah's Witnesses are not traditional Christians. I don't speak for them and I do not speak for anyone who says that I do describe their concept of the Holy Spirit.
Who do I think I can speak for? I think I can speak for Orthodox Christians, Catholic Christians, and some Protestant Christians, when I say that they (i.e. the traditional Christians) acknowledge the existence of the Holy Spirit and believe that the
Holy Spirit is the third Person of the Trinity.