Approach this question with an abundance of caution. You’re going to be working with a lot of people in your new role, and you won’t get along with everyone. But you’ll still need to work together productively.
The “who” here isn’t important—and no matter what, do not give a specific name. Rather, the interviewer wants to know how you managed to work with this person despite the difficulty. After all, if you worked through a challenging work relationship in the past, you can do it again. Don’t be petty and be sure to end on a positive note.
I had a challenging lab partner last year. He was disorganized, and we kept missing deadlines because he didn’t update me on his progress. Eventually, I insisted on weekly, in-person check-ins to stay on track. He found it annoying at first, but we got the work done, and in the end, we walked away on good terms.