[Kaijo]

Spoiler for manga spoilers...:

One of my favorite characters, although she seems to be kept "dumb" as her powers could really make her a powerful S-rank if she could bring out multiple spirits and use them smartly (combined with that spell she used in the manga).

With the recent manga arc, it looks like she'll get the Capricorn key, which leaves 2 keys left: Libra (usually shown as weighing scales) and Pisces (which is two fish). So there are at least 1 or 2 other arcs involving keys coming up.

One particular thing I thought recently, is that there is technically a 13th zodiac sign: Ophiuchus, also known as Serpentarius. It might be an interesting trick to pull, having Luzy discover the 13th key after she gets the other 12.