Communication skills
Most importantly, if you're considering a PR career you will have to be a good communicator. You will not only need to be a confident speaker, but an excellent listener too. This doesn't necessarily mean you will have studied for an English or foreign language degree, you could be a scientist or technologist who has a great knack for explaining complex concepts.
A PR professional has to be sensitive to subtle nuances in language and culture because the job will involve communicating with a range of people across different mediums (social media, in person, on the telephone and in writing) across different territories. You will need to be able to appreciate other people's priorities and pressures.