As a minority, you can't assume that a union rep is on your side. Especially if the complaint is against someone else at your company who is represented by the same union, if that person is more politically important (within the union) than you are, you're risking a lot - up to and including retaliatory termination of union membership[0] by speaking out. It's your word against theirs, and you're not the one hiring your representative, so you can't be sure that their incentive is to advocate your case to the bitter end, instead of to brush things under the rug.

If you want someone who has no conflict of interest to advocate your case, you need a lawyer, not HR or a union representative. At best, the latter will refer you to a lawyer (at which point they're not doing anything for you you couldn't do yourself). At worst, they will cost you your job, and possibly your career[1].

[0] Which, incidentally, is not protected by the same laws that protect retaliatory termination by employers.

[1] If you're in an industry that's represented by one single union, having your membership terminated means you should probably start looking for other career options.