You know them. They've been in the promotional material for a few months now, ingame since last year, and on the website since Duneoneering's release. They're the signature heroes, and for all we've seen of them, we haven't actually seen them in action. The main three, Owen, Ozan, and Ariane, have appeared in their own dungeoneering saga and very briefly in the Troll Invasion tutorial, while the Raptor, Linza, and Xenia play minor roles in short quests, but that's the extent of their involvement in the game. These characters have well-developed personalities and roles, and they're being wasted.
The most noteworthy example of this is Owen, the warrior of the main three heroes and a Temple Knight agent. Players have the ability to join the same organization, and we've been able to work with them in a number of storylines, including both the Void and Mahjarrat series, but Owen himself has been noticeably absent. He was even supposed to be the subject of his own quest, which was ultimately cancelled. Neither Ozan or Ariane have had as much of an opportunity to make an appearance, though they have appeared far more frequently in the promotional material. Ozan in particular seems to be the new face of the game, given his presence on the splash page and being the focus of both the cinematic trailer and the commercial derived from it.
It seems silly that three named, fleshed out characters that have dominated the game's artwork and website for almost two years have never made a significant appearance. As of this week's update, they've played a smaller role than the other three signature heroes, who have not had nearly as much exposure. Xenia filled the role of the player character's mentor before the signature heroes were announced in The Blood Pact, while Linza loaned the player her smithing skills in Deadliest Catch and The Raptor actively competes against us in Song From the Depths.
Similarly, some minor characters from other quests receive far more character development than their significance in the game would suggest. These characters rarely reappear, though there is plenty of potential for them to do so in the future, even alongside the signature heroes. For example, Eva Cashien was introduced in Salt in the Wound, during the quest the player has the potential to discover more about her backstory and personality than most other significant characters in longer quests, and she has not been seen since. As she is a Temple Knight, she could reappear any time that organization is involved with a quest, but did not appear in Ritual of the Mahjarrat alongside other agents (Including Tiffy Cashien). The members of the Thieves Guild in Buyers and Cellars also have the potential to reappear in a future quest, possibly alongside Ozan. Finally, Ariane can potentially reference one of the many quests in which you discover a new kind of magic, and it would only make sense that she makes an appearance in a future quest centered around those discoveries.
Jagex has proven that they can connect seemingly unrelated pieces of the world and lore into a consistent whole, and they've had plenty of opportunities to do so with characters that they've created almost entirely for that purpose. Instead, the signature heroes are examples of some of the issues that plague many pieces of content; they don't live up to their potential and there is no sign that they will at all, in spite of the many hints and promises to the contrary.