Luke Fon Fabre is a noble who, for the last seven years since he was ten, was locked in his manor in response to an incident where he was kidnapped by a neighbouring kingdom and lost all of his childhood memories. Though he was re-educated by his mentor, Van Grants, the constant attention he was given by the servants of the manor as well as his parents turned him into a spoiled, egotistical jerk with little knowledge of the outside world until he is once again taken from his home and forced to journey through enemy territory in order to make it home. It is during this journey that he encounters Asch for the first time, who despises him and actually seems to harbour the intent to kill him despite orders not to.
With the help of the rest of your party, Luke makes it back home, only to be sent out again several months later to a mining town plagued by a deadly poison. Though he has had some character development, he is still quite egotistical and this leads him to strike a secret deal with his mentor to use his special powers to erase the poison and save the town, thus making him a hero. Once he gets to this town, Luke carries out the plan despite many pleas by his party not to, but only when it's too late does he realize he was being used by Van, and that his special power, rather than save it, actually wiped the town from existence along with its inhabitants, thus causing him to emotionally break after the party rejects him.
Very soon after, Luke is confronted yet again by Asch, who reveals that he is actually a Replica (An exact genetic clone of a person or object.), and that Asch himself is the real
Luke. It's at this point that Luke completely snaps, and goes into a coma. Upon waking up, he realizes how much of a jerk he was being during his journey, and decides to atone for his mistakes and change himself for the better.
After this point in the story, Asch becomes more of an anti-hero, as he too is working to stop the antagonist, Van, from replacing the entire world with Replicas, but he does not actively help the party. This is because he hates Luke for ruining his previous life, and this causes him to reject Luke despite his numerous efforts to change himself and apologize to him, saying he can never be his own person. The subplot between them reaches a final climax in the final dungeon when the two are trapped in a room where the only door out must be manually held open. Luke insists on staying behind, reasoning that Asch, the original Luke, is stronger and more capable of defeating Van, but Asch, not wanting to be pitied, challenges Luke to a duel to settle who will face Van.
Luke wins, and Asch lends him his weapon to fight Van, leaving him defenceless against a group of soldiers. he manages to kill them, but receives three swords through the chest, leaving him mortally wounded. Right before he dies, he tells Luke it's "Up to him", implying that he finally accepted Luke as his equal.