In my opinion, every dream character can act as a guide. They can all teach us something and make us better LDers and dreamers in general.
There are particular dream characters that may show up repeatedly, whom you make a connection with, and learn tons of stuff from. That is what I think of when I think of a dream guide.
The only way to really find out I would think is to have many experiences with the same dc over time and determine based on that.

When I was a kid, I learned how to lucid dreaming naturally because of nightmares. After becoming familiar with the dream world, a dream character came to me and started teaching me things. He didn't say "hi I am your dream guide". It was a relationship built over time and experience. He taught me things about LDing that I did not learn in waking life. At 7-9yrs old, I had never heard of lucid dreaming.

Just explore your dream characters. Talk to them, and explore the differences. Some dcs are zombies without a brain, and others seem smarter than you. So explore, interact, and you will find a dream guide eventually.