Oliver Cowdery

Oliver Cowdery was a school teacher in 1829 and he taught some of Joseph's younger siblings. As customary in the times, Oliver stayed with some of his student's families, and so he stayed with the Smith family for a time. During this time he heard a lot of talk about the Book of Mormon, so he asked Father Smith about it. As he learned about it, he was told that Joseph needed someone to write from him as he translated. Oliver soon wanted to help in this effort.
The two soon became friends and Oliver gained his own faith in the teaching of Jesus Christ given through continuing revelation. From this point on Oliver helped to transcribe the Book of Mormon, including trying to translate for a time as well. He was unsuccessful in his attempts to translate however, and this was discouraging for him, but he continued in his role as a scribe for the prophet.
Soon after, in the same year (1829), many were wondering about baptism and how to receive the one true baptism that was needed for salvation. Joseph and Oliver came to the Susquehanna River and inquired of God to know where to find this truth. It was then that the resurrected John the Baptist came to them and taught them of the things they needed to know, as well as giving them the Aaronic priesthood with the keys to baptize others. Then later that month, both Joseph and Oliver were together again, inquiring of the Lord, when they were taught about and received parts of the Melchizedek Priesthood from the resurrected Peter, James, and John (who were three of Christ's original apostles).
Oliver Cowdery's main role in helping the church develop is debatable, but the main points that are important to understand is how he helped Joseph as a friend, supported him as a scribe, and had faith in Jesus Christ to understand the different priesthoods.