Who is Alma?

There are actually two Almas in The Book of Mormon: Alma, a father, and Alma the Younger, who is his son. Both live on the American continent around 100-70 BCE as prophets and government leaders. The first Alma starts out as a priest for a wicked king named Noah. After another prophet, named Abinadi, preaches against King Noah, Alma escapes the palace and starts preaching the gospel of Jesus Christ in exile. He later rises to become a high priest for a good king named Mosiah.
Alma the Younger, however, starts out as a rebellious young adult who not only disagrees with his father's religious beliefs about a Savior who will come to redeem mankind, but he outwardly disgraces him and the church. In fact, Alma the Younger becomes friends with King Mosiah's three sons and they strongly persecute members of the Christian church. Not only that, they start to lead members away from the church while also behaving poorly and harassing others.
A beautiful story of redemption unfolds as Alma the Younger sees an angel who puts him in a coma for three days. During that time, Alma the Younger suffers sadness, sorrow, and pain for his sins. He sees Jesus Christ in a vision and decides to repent and to change his life for the better. He comes out of the coma with a repentant heart and does everything he can to make amends for the wrongs he committed in the past. Later in his life, he leaves a government position as a judge to serve a mission to preach the gospel of Jesus Christ.
Alma the Younger's conversion story is found in chapter 36 of The Book of Alma, of course found within The Book of Mormon.