The spirit guides are also known by others as the guardian angels, they help to guide us in making the right decisions in life. The greater part of what we know as our conscience is made up of the spirit guides putting thoughts in our mind for direction.
But what are this spirit guides? Are they actually angels? The truth is spirit guides are not angels but spirits that are developed enough at least to some level to guide still incarnate spirit on earth (human).
The spirit guides are not suppose to be looked at as some very evolved or developed spirit guiding men on earth but they are develop than the spirit they guide but not like HIGHLY developed than the spirit they guide. The reason for this is for communication to be possible between the two. Imagine a professor asked to teach a primary school children, it should be easily observed that no matter what the professor is saying he wouldn't be communicating at all, just a waste of time. This principle also applies here, for effective communication to exist, the spirit guide shouldn't be too develop than the human he guides.
This is way spirit guide can NEVER be angel, the distance is just too much. This kind of error also makes some people to think that their guide is even an archangel like Gabriel or even God Himself, why hire a professor to teach what a primary school teacher can teach effectively? The law now is, the student must not know more or develop than the teacher, this means that as you develop so will your spirit guide be changed to meet your needs. So basically, if you're a kind of person that thrives to develop himself then you can't possibly have one spirit guide throughout your life. So if you want a very advanced spirit guide then develop yourself to meet the standard because it can never happen any other way.
Now, where do these spirit guides come from? To teach effectively you must understand the nature and disposition of your student, and who could fit this criteria than a fellow human? And this is also the reason why an angel can not be a man's guide because they don't know what it feels like to live on earth just like a rich person can not know what it feels like to be poor, the best he can do is speculates.
So yes, spirit guides are "dead" humans who maybe has made some mistakes and now here to guide the still incarnate spirit not to make the same. And yes your grandfather or mother could be your spirit guide, even friends and relatives if they are fit for it.
It's important to know your spirit guide is not infringing on your freewill, the best he can do is put the right thought in your mind as your conscience, you make the final decision so the consequence is yours and yours alone.

logically correct Buh no, "for I go and send the comforter to yhu and he will teach yhu all things" blah blah blah, its a biblical verse, the Christian equivalent of a spirit guild is the holy spirit, a spirit guild can be of any level, a spirit guild can very well be a human being, an animal or pure spiritual energy, most times the term spirit guild is associated with spirit,pure spirit, Buh term is actually use in reference to a spiritual journey, its the spiritual journey that gives the term spirit guild its meaning, a human more experience in spiritual matters can guild yhu on yhur spiritual journey, that human is automatical yhur spirit guild.

not all human are on the same level spiritually.
its important to note that yhur spiritual guild is a representation of what yhu going through, yhur journey spiritual, a spiritual guild is not a static notion but a ever changing one.

for example most people can't get their spirit guilds to give em a static name, personality, form etc . working with the spirit your spirit guild yhud find that sometimes they contradict themselves. what is appropriate for today based on yhur spiritual level n growth might b inappropriate tomorrow.

"do not call what I call clean, unclean " The Abrahamic God said that, after initially declaring certain animals, race and people unclean.

its like asking a four year old to understand university stuff.

#its only human that think of God as a static being, yet as human we've been known to evolve, not just scientifically, Buh inner evolution, if we are in the image of God,then the God must evolve too, "the All is one and the one is all" Maxim .