It isn't someone specifically. It's the tradition of the Church that is supposed to guide the bishops and priests. I mean, you don't need someone to tell you what the Bible says concerning many things, but with certain ambigious or easy to misinterpret verses, I sure do my trust in the teaching authority, either through the clergy, or through researching the sources the Church relies on. The whole "Holy Spirit guides the individual Christian to interpret Scripture" viewpoint is just relativism, given a nice face.
So 2 Pet 1:20 means what to you?
Not all that nice, I might suggest.