Cain in the biblical story was Adam and Eve's first son. He was known for his offering to God which was not well - received compared to his brother Abel's. His jealousy towards Abel led to the first murder in the Bible, and after that, he was condemned to be a wanderer on the earth with a mark from God to protect him from being killed.
David was this young lad who was taking care of his father's sheep. He was not from a military background but was chosen by God. In the story against Goliath, he showed remarkable bravery. He trusted in God's power and his own skills, like his ability with a slingshot, which ultimately led to his victory over the giant.
In the bible story, Cain and Abel were brothers. Cain was the eldest son of Adam and Eve. He was a tiller of the ground. Abel, his younger brother, was a keeper of sheep. Their different occupations led to them making different offerings to God, which in turn set in motion the events that are so central to their story.
I'm not sure who David Fuller is in this story. There are so many people named David Fuller. He might be a character who had an important role in a particular event or a person with a unique personality in a family or social group. But without more context, it's difficult to be precise.