First of all, and this should almost go without saying, they need to great communicators and have a patient, friendly demeanor. They should also be able to write flawlessly without making mistakes with spelling and grammar, or you'll risk looking very unprofessional. People are usually quick to point out mistakes like that on social media.
It's also a good idea to look for a person or social media digital marketing agency with some previous experience in your niche (or at least in a similar niche). Otherwise they may find it difficult to relate to your audience and post things that are interesting to them. It will also make it harder for them to answer any questions your fans and followers might have. Of course they can always relay such messages to you for a response, but that will take time out of your busy day and somewhat reduce the usefulness of having a social media manager in the first place.
Finally, be on the lookout for complementary social media digital marketing agency skills that could be useful for a social media manager to have. For instance, if they have the ability to edit pictures it will make posting new content a lot easier as they can adjust images as necessary. Same goes for creating and editing videos. You'll also want someone with great research skills, so they can easily go out and find new, interesting content to post.
These are just a few of the things you should look for in a social media manager. You should put some thought into what would be important for your particular business and situation, as it will be different for everyone. Take your time to find the perfect candidate instead of rushing off with the first one you stumble over - it will pay off in the long run to be diligent!