Man, I've seen a lot of confusion about this one online because there are a few 'Elora' titles floating around. If we're talking about the fantasy series that starts with 'Elora of Stone', then the key character is definitely Elora herself, this apprentice stonemason with a mysterious past. But the whole 'key' thing gets tricky because her mentor, Garrick, is arguably just as important in the first book—he's the one who unlocks her abilities and shields her from the Temple's seekers.
Honestly, the second book, 'Elora of the Deep', shifts focus a bit and introduces the Sea-Singer Kaelen, who becomes a pivotal figure in her journey to control the elemental magics. So while Elora is the protagonist, the key character might change depending on which conflict you're looking at. For the core mystery of her origin, it's her; for the magic system, it's the mentors and antagonists pulling the strings.
The fan wiki has a whole debate thread on whether the 'key' refers to narrative weight or plot-device status, which is a bit much for me, but it shows how layered these books are.