A constant complainer can drain you emotionally and make you lose motivation. Here are ways to deal with a complainer so you have a more efficient rather than exhausting environment.

Make sure you are the right person to hear their complaints. If they are
complaining ABOUT someone, let them know that they need to talk WITH that
person if they want the situation to change.

Really listen! The person complaining may keep repeating themselves because they feel like they are not being heard. Show you are listening by reflecting back what they have said and asking applicable questions.
If appropriate, give brief advice.

Call them out. If the complainer continues to complain and you’ve already listened and engaged, tell them that you have heard their story and there is nothing else you can do. Perhaps they need to take it to someone who has the authority.

Redirect. If the conversation starts to go into “complainer land,” change the topic! This can switch the tone and stop the complaints.

Avoid them. This sounds obvious, but sometimes we forget that we have the
freedom to walk away, get some fresh air and return to doing our work.

It might be hard to deal with someone like this, but following these steps may make your life easier, and perhaps redirect the complainer in a
better direction as well.