Mordred

Mordred was a druid boy who was either born a druid, or selected for his magical abilities to become an apprentice of the druids.

He first appeared in series 1 episode 8, "The Beginning of the End", when he was journeying through Camelot with the druid Cerdan, possibly his father, to collect supplies. The supplier betrayed them to the guards, and Cerdan was arrested, but Mordred escaped, though he was wounded. He called out in his mind, and Merlin heard him, and smuggled him to Morgana's chambers. The boy called him Emrys, and Merlin discovered this name for the first time.
Shortly after this, his (possible) father was executed.

  Morgana always felt that that she and the boy had a bond, and for this reason, she did whatever she had to do to help smuggle the boy out of Camelot, and eventually, with Arthur's help, they succeeded, and Mordred was returned to three druids, the leader amoung them.

Leader of the druids

From then on, Mordred lived in druid camps. He either was taken strait to the druid camp in the forest of Ascetir and the leader moved on to a different camp, or he lived in the druid camp which lies in Cenred's kingdom (where the leader dwelt for a while during series 3) before he lived in the forest of Ascetir.

Whilst he grew up in one of the druid camps, he would have met Kara as a child, and they became friends. Of this friendship, or of where they met, I cannot say much more, as I only have information from what was mentioned in episodes, but Mordred cared for Kara deeply.

After about a year, Mordred met Morgana again, when she came to the druids in search of answers about her magic (series 2 episode 3, the nightmare begins). She had hurt her leg. Mordred was able to care for her "as you did me", he said. Morgana felt at home there, and didn't intend to return... but Merlin had followed her, and Arthur and his men had followed his trail.

They had to run, as Arthur's men attacked the druid camp. Morgana passed out because of a wound to her leg, and the leader of the camp, Aglain, was killed, but Mordred escaped.

Alvarr

He wandered in the wild until a man called Alvarr found him. Alvarr had magic, and was the leader of a band of renegades sworn to bring down the king, possibly taken over from Tauren, who was killed in series 1 episode 12.
  When Alvarr found Mordred and discovered the boy's power, he hoped that Mordred would be able to master the crystal of Neahtid, which was locked in the Camelot vaults. He used Mordred to get to Morgana so that she would help them steal the crystal, which she did, but not unnoticed by Merlin.
  Shortly after the crystal was captured, Alvarr's camp was raided by Arthur's men, and the crystal recaptured.
  During the raid, Mordred tried to run from the battle, but Merlin used his magic to trip him up, and two of the Camelot soldiers came up to him. Mordred used magic to throw two spears, and killed them.
  He turned to look at Merlin, and in his head, he said:

Then he fled from the camp, and we didn't see him again for a long time...

I often wonder what would have happened if Mordred was told that it was his destiny to kill Arthur right after Arthur helped to save him, or when he was a loyal knight of Camelot. Would he have wanted that? I don't think he would have. Would he have been able to change his fate?

I doubt we will ever know.