Two are better than one, because they have a good return for their labor: If either of them falls down, one can help the other up. But pity anyone who falls and has no one to help them up. ECCLESIASTES 4:9-10
It is an informal relationship between two individuals. Your mentor will do as many things ideal for your career and life as a whole, which your coach or trainer would also do, but he is not bound by any legal obligation for it.
You cannot appoint a mentor. This type of relationship develops over time from a professional association or friendship. Over years, your relationship with your mentor intensifies. In most cases, the mentor shares his experience and knowledge with you in an informal way.
You can consider anyone as your mentor whom you consider successful and desire to be like one. Your mentor guides you through all ups and downs of your life and over time your relationship with the mentor intensifies and can even transform into a long-lasting friendship.