I don't know how much you can really decide about your dream guide. Dream guides are (depending on your views) either representations of your subconscious mind or spiritual beings here to guide you in lucid dreaming, and sometimes other aspects of your life. Therefore they, almost by requirement must fill a specific role; act and look a certain way to convey their message to you. If you want to try and change them go ahead, but I wouldn't recommend it.
Also, dream guides will act as if they have a conscious mind by default. This is a good way to pick them out of a crowd. They are much more intelligent than random dream characters.

If you want to create a mail-order dream character, then, so far as I can tell, this is totally do-able. IWL, you should establish what this character is. Draw pictures of them. Write out stories of how they would react in certain situations. You need to know everything about them, as if they were someone really close to you like a best friend. The more you know this character, the better.
Once you know this character like the back of your hand, you can use either autosuggestion or a summoning technique (or both) to bring them into your dream.
If you have done a good job, the dream character should take on a life of their own, and should be conscious just like a living, breathing human being.