Was urged to join the Templars like his best friend Cecil, but chose the path of a Grey Warden like his father, instead.
A child bully, enjoyed beating up Cecil as a boy
His father died of poison when Kain was a young teen, he and Cecil were raised by a nobleman
Warrior with reaver specialization
In love with a spirit healer: his other good friend, Rosa, who is in turn in love with Cecil instead.
Accidental killing of a village: Joined Cecil on a Templar mission which ended badly, in the deaths of all the mages there (they'd been summoning demons- or at least accused of it), argued with Cecil but they saved the sole survivor, Rydia
Kain was mind controlled by powerful blood mage, Golbez, who was in turn possessed by a demon, Zemus. Under the influence of this magic, did terrible things, including betray his closest friends.
Goes back and forth between betrayal and loyalty, stealing important plot items from the party
Banded together with a group of fighters to fight for a mysterious leader, Cosmos... they fought constant battles against darkspawn, the demon Chaos, and the fighters who Chaos had corrupted to serve him
After an especially dangerous and difficult battle, Kain was gravely injured and stricken with an infection. He spent over a month slowly wasting away, tended to by healers, including Inessa.
Kain eventually died of the infection/injuries, with Fenris arriving to console Inessa (this Bad End AU is canon for the the Fenris/Inessa AU)