A mentor should consist of 3 things
1. Have a track record, be knowledgeable and connected in your the area of your startup (e.g. real-estate, fashion or semi-conductors, building a startup, writing a book)
2. Share your vision
3. You should think he's cool and he should think you're cool
If you have all of these 3 things, then the mentor is valuable and you can think about giving a stake in your company.
Definitely sharing vision is a big one. I would also add someone who shares similar values and ideally who's done what you're looking to do. Finding the right mentor takes lots of time and effort, and having the right mentor will make the journey a lot easier.
There are a few factors that you need to consider when looking for the right mentor.
1. Find someone who has demonstrated success or competence in the areas that you are focused on.
2. Find someone who resonates with you.
3. Ask others for recommendations
4. Look for samples of the prospective mentor's work (e.g. blog, Youtube, etc...)
5. Be completely clear on what your expectations are for the mentoring relationship.
Schedule a call if I can assist you further.