Can I ask for recommendation from a person that I am working with?
I know this question was asked a while ago, but I found it while searching and can't believe it wasn't answered.


  My mentor and I helped his team to analyze their data in that time and the project is still going on and sometimes, if his team needs, I work on their project which is not paid.


It sounds like you helped in research and also have a personal relationship with him. Of course this is exactly the kind of person who can probably write a great recommendation letter or give a great reference, especially if he liked your work. Even if he isn't in exactly the same field as you, he can probably write a very specific recommendation letter pertaining to research you helped perform.