My idea was based loosely on the way a Dalai Lama is chosen as a child, becoming the spiritual leader of Tibet. The back-story concept I'm working with is that there were originally multiple warring clans on the Island, but their constant wars were depleting their numbers and damaging the island ecosystem, threatening to lead to their ultimate extinction (basically, a microcosm of what happened with Earth as a whole and the major nations that fought in the War). The old shaman/Commander had a vision (open to interpretation, as such things usually are :) ) of a Chosen One who would come and fix things.
When Esme came, the old shaman brought her forward as the hoped-for Chosen One, and brokered a peace between the remnant clans, unifying them into the Azure Waters Clan (name chosen to emphasize the waters surrounding the Island, and hence, the unity of the surviving people surrounded by them). Esme has since sought to strengthen the unifying ideology by pointing to the Consumers (as represented by the Ark) as the common enemy. They're the perfect enemy because their threat is undeniably real (the Ark is visible crossing the night sky, and "their kind" destroyed the world) but they're also not there to fight (so, no destructive war)...until the 100 show up.
Esme became the nominal leader of the new, unified Clan under the old Shaman's guidance, with Rhea as Esme's designated successor. The old Shaman could (probably would) have also been a more conventional Commander, but since her vision, she'd been more intent on promoting the hope of the coming Chosen One. Kind of a Ghost Dance/cargo cult sort of thing, until Esme turned up in seeming "fulfillment."
So, Rhea would have grown up expecting to become more of a conventional Commander, until her predecessor's vision, and later Esme's appearance shifted the ideology. While the old Commander/Shaman trained Esme, she and Esme formulated and promulgated the new ideology of unification under primarily spiritual, rather than primarily military leadership. For four years, Esme was like a child Dalai Lama or Pharaoh, nominally in charge (due to Chosen One status) but de facto under the regency of the old Shaman/Commander. When the old Commander passed, Esme fully took her place.
Esme has been trying to cement in place a "we don't kill our own" ethic, while directing the Warrior attitude outward toward the Consumers.
The idea behind all this is to reconcile the more canon-like character bios of the other Grounder characters with the existence of the differences I introduced. I hope it works. :)