Seth is a knight in Rivellon.
Seth is found in Divine Divinity battling orcs near Aleroth. It is Seth who alerts the village of Aleroth about the war. Seth is very much fond of his weapon Brightblade, and when he introduces himself to you, he also introduces his weapon.
In Divine Divinity, Seth is also introduced as a general as he is seen leading a few soldiers before entering the village of Aleroth after they are slain by the orcs outside. After warning you of the dangers outside, he returns to the gates of Aleroth to defend it.
Once you inform the army of Aleroth's plight, Seth vanishes Aleroth with his duty of defending it complete.
It's possible to kill Seth in Divine Divinity by using the poison cloud spell. Upon his death he will drop the Brightblade sword which statistics will vary (in other words, the statistics are random) when Seth is killed.
Interestingly enough, Divinity 2 is set decades after Divine Divinity and despite this, Seth appears as a youthful man in Divinity 2. This could be due to him actually being knowledgeable in the magical arts which can extend one's life but he never actually displays any magical knowledge which raises questions.