I just started researching this fall semester, and I've never actually met the PI. I just work with the research fellows in the clinic. Should I get a LOR from the research fellow? I already have an LOR from a very close advisor.
If you just started research recently, why are you looking to secure a LOR already? I'm assuming that you aren't applying this cycle, so you wouldn't need a letter of any sort until next summer at the very earliest - why not wait it out, get to know the PI, and then ask for your letter?
As an undergraduate, the point of research is to show that you are making a meaningful contribution to the lab's projects. If your PI has never met you, or talked with you about your progress, I can almost guarantee you that you haven't made that contribution yet.