If the being that has the job of being the always-loser is not another regular soul, but one of the divine beings of the plane, that being could easily be one that positively enjoys always losing debates, either because as a being of pure law and goodness it sees the fulfillment of its duty/job (ie to lose) to be maximally satisfying, or its psychology is such that it simply enjoys intense intellectual discussion regardless of who "wins".
Basically, always losing at a debate sucks of humans and those with human psychology. But there is no reason why Celestia needs to employ such beings for that job.