"If this marriage were only a way to take the throne, I'd have killed him a long time ago." - AllenAllen Schezar and Dryden Fassa.
Allen and Dryden's opposite personalities cause them to clash from their first meeting. Whereas Dryden is more realistic, free and easygoing, and doesn't take everything so seriously, Allen is an idealist, more proper, a stickler for the rules, and operates by the strict moral code imposed by chivalry. Allen's resentment of his father prevents him from taking any genuine interest in the mysteries of the Mystic Valley and Atlantis that Dryden and the others wish to know more about.
Dryden begins to read from Leon's diary. Allen is especially frustrated with Dryden for not taking the whole situation as seriously as he does, likely because Dryden does not realize that Allen's anger is not due to a difference in personal values or lifestyle, but rather because he believes his father abandoned his family. Dryden's professed enjoyment of the tale and admiration for the way Allen's father lived his life pushes Allen over the edge, and before he leaves the room, he declares they probably won't be friends, which Dryden takes as a compliment. Direct interactions between the two mostly end after this point, and it is unknown whether Allen forgave Dryden for misinterpreting his feelings.
Although not covered directly in the series, Dryden's betrothal to Millerna likely caused tension between Dryden and Allen as well. This would have been caused less by Allen's existing relationship with Millerna and more by his concern that Dryden wouldn't be the best man to lead Asturia. When he is in the graveyard with Eries, Allen accepts her belief that Millerna has found happiness, and says that Dryden is a good man and that he wants what is best for Millerna as well.