The investigator should be only seeking the facts and remain objective, so it is surprising that he/she has made comments beyond this at this point.
If this hearing is in fact an investigatory interview, you can ask to be accompanied.
I'd suggest you stay calm, and prepare the facts of your side of the storiy/stories. Facts are your best defence.
Be ready to provide context, and facts.
If you think someone has a grudge against you, be ready to say why, but again stick to facts as far as you possibly can.
Try to co-operate with the process as that will give you best chance of showing your side of matters.
Ask for the draft written report of the interview as you should be asked to sign it to confirm it accurately refelcts what you said.
Try to stay calm - if you feel upset or nagry, you can and should ask for a break to compose yourself.
Be ready to answer the Q 'Is there anything else you'd like to add? If you are not asked this, say that there are things you'd like to add that you have not had the chance to say and get these points across.
Be ready to consider others points of view too - is it possible you have caused offence for example, without meaning to do so? Sometimes it is is helpful to put yourself in the other's shoes and see the situation from another angle before you reach a conclusion in your own mind.
If things have gone wrong in a working relationship, the manager considering the investigation report will prob behave more sympathetically towards you if you acknowledge you may have contributed to the problem (if you have) and sometimes people cause upset w/o intending to. It can be useful to acknowledge this type of thing and accept you need to modify behaviour.
On the other hand if it is a group who have ganged up against you, you will need to be able to demonstrate that this is what is happening. The investigator should ask who you would like to be interviewed so you could propose other staff who know your work well and would speak up for you and your work. You could also propose staff who have witnessed what has been happening.
Hope this helps.