My feeling from the in-game lore was that they’re not really itinerant adventurers like the Battle Chasers are, since they’ve both got businesses that they run. I could see there being something of an origin story (explaining how they got where they are now, since they’re the only beaskfolk on the island), but a “further adventures” story would require something significant to happen to their existing lifestyles.
I disagree with the assessment that Lyko is a more capable combatant than Raha. Lyko focuses upon throwing hordes of enemies at the players (and “Arena Master” is less about being the best fighter in the arena than it is about administering it) while Raha focuses on throwing extremely powerful and difficult to find foes at the players. Based upon their professions, I’d probably say that it’s a wash as to which is the more capable combatant. I would guess, however, that they’ve both managed to do something significant with their previous lives since Raha has managed to get a a fair amount of respect within the town and Lyco managed to get enough money to put together an entire arena in the middle of the wilderness. It’s also important to note that all of the stuff that Raha sells in her store is supposed to be stuff that she got off of enemies that she has killed while out hunting recently, which suggests that, at the very least, she’s as good of a fighter as the rest of the party. I’m not sure about Lyko since there isn’t really any suggestion as to how he fills the arena.
Their combat styles, based upon their sprites, would seem to be agility based (due to the lack of comprehensive armor). Lyko dual-wields swords (as evidenced by keeping one on each side so he can draw them both simultaneously) while Raha uses a spear or polearm of some kind.
Iirc, they’re both Beastfolk (?), as you learn from speaking to them. It’s a distinct species/race from the Lycelots. No clue about their cultures or rituals beyond the fact that they both seem to prefer primitive “tribal” architecture, even in nominally civilized areas. Presumably there are more than just the two of them, but they’re likely not native to the island, since I get the feeling that Lyko and Raha are both immigrants to the isle.
As to the relationship between Lyko and Raha, iirc, they’re brother and sister, since I recall Raha telling you to visit her brother in the Arena.
I could definitely see Lyko and Raha starring in a gaiden game or potentially joining the other Battle Chasers as additional party members in a continuation, assuming they have a reason to abandon their current “businesses”.