Arn was the name of two characters from Hal Foster's comic strip. The first was the prince of Ord, Valiant's rival for Ilene's love, who gave him the Singing Sword (Flamberge) during the pair's attempted rescue of her and who become his friend (and, later, his firstborn's godfather). The second was Valiant's first child and namesake of the first, who featured prominently in later strips. Arn was Foster's original choice for the strip's protagonist's name since it would have been a common Norse name at the time of the setting, but the members of the strip's publishing syndicate agreed that Valiant sounded better for the hero.
The show's infrequent use of fantasy elements outside of dreams and its portrayal of Merlin, Morgana, and others like them as alchemists often mistaken for magicians is because at the time of its airing, the Family Channel wanted as few of its shows to involve magic as possible. Interestingly, this resulted in it having much in common with Hal Foster's comic strip on which it was based since in both a witch (not called so in the show) appears in the first story arc and some beasts there and near the Misty Isles, but magic appears less as the story progresses. The one clear exception in the show is the Singing Sword still having its famous power. Excalibur's being drawn from the stone, Morgana's and the witch's concoctions causing a specific dream or nightmare for those exposed to them, and Merlin's possibly being able to see or act through a falcon are left ambiguous as to whether they're actually magic.
Rowanne, Sir Bryant, and Denys are all completely original characters, but each could be interpreted as having at least a few similarities to ones in Hal Foster's comic strip. In the comic, Valiant and Arn both love Ilene, while in this show they are both shown to have feelings for Rowanne (albeit the two reflecting on this is temporary). Though not a Moor, Sir Tristram is one of the earliest knights to befriend Valiant and often seen with him and Sir Gawain, very much like Bryant. Although not related to anyone noteworthy as Denys was, Geoffrey also found himself free from a cruel home situation and became the most developed of Valiant's squires.