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.