If I don't ask for a recommendation letter from a postdoc I did research with, will it look bad? I am applying for math graduate school. I have two people that I did research with: A professor and a postdoc. I have a strong connection with the professor, but the postdoc I do not. He was very passive aggressive with me and our relationship does not seem to be a good one. I do not want to ask him for a letter. However, I am wondering if this will look suspicious or hurt me in any sort of way. If I say in my application that I did do research with a postdoc but I did not receive a letter from that postdoc, will that affect me or the admission committee's decision? Will the admissions committee likely email this postdoc to hear about me from him? 

Thank you very much.
Admissions committees expect letters to be signed by professors.  It is perfectly normal to get a letter from the professor who supervised the postdoc.  The only case where I would expect the postdoc to even be mentioned in your application would be if you published a paper with the postdoc which did not include a contribution from a professor.

Keep in mind that professors often ask postdocs to help them write letters of recommendation.