What is the role of a mentor in junior software engineer's career? Also what makes a good mentee?

In my opinion a mentor should be able to give good advices to the mentee as a master of the subject and justify his points. For instance, in the choice of the stacks for a particular project. I mean that the mentor should master what he is advising or at least been able to reference someone else if he don't master the subject.

A good mentee challenges the mentor. Not in an aggressive way of course, but in a way that means the mentor grows as well. The 'mentee' will ask good questions that really makes the 'mentor' think. In that sense, a good mentee also acts as a mentor themselves. If you're interested, this is an idea that L. David Marquet discusses in his book Turn the Ship Around. He calls it mentor-mentor, as opposed to mentor-mentee.

Great question! Mentors are so important in all areas of life -- both having one and being one.