Imagine a setting where anyone might have an inner self. Another aspect to their personality that remains hidden. When it rises to the surface, it can endow the owner with mysterious powers.
Those who have the potential for this ability but lose it are left listless. Their lives no longer have any purpose, and they become almost soulless.
No, this isn't Persona... this is Shugo Chara!
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"All kids hold an egg in their soul. The egg of our hearts, our would-be selves, yet-unseen."
Everyone has a Heart Egg. The Heart Egg represents what you want to become; your ideal would-be self. Many people grow up without ever realizing their true dream, and their Heart Egg becomes dormant. But for children, there is still unlimited potential, and the Heart Egg is strong.
Those with particularly strong Heart Eggs may manifest them physically. When this happens, it's only a matter of time before the egg hatches into a guardian spirit called a Shugo Chara. These Shugo Charas represent an idealized view of what you might want to become. For example, Tadase has always been shy and bad with people, so his Shugo Chara, Kiseki, is a king: brave and bold and not scared of anything or anyone. So what if he might also want to take over the world...
Notably, Shugo Charas are invisible to most people. Those who have a Shugo Chara themselves are able to see them, as well as those with very powerful spiritual sense. Some can hear their voices but not actually see them. Young children, notably those who are too young to have decided what they want to become, can also see them.
On their own, a Shugo Chara has very limited abilities. They can't do much short of flying around, really. The real magic comes with the Chara Change. When a Chara Change is initiated, there is usually some very minor physical indication (Tadase sprouts a tiny crown, Amu's hairclip changes shape, Rima adds a bit of makeup). There are two bigger changes. First, your personality shifts toward that of your would-be self. This is usually involuntary, and Shugo Charas tend to use Chara Change whether you want to or not.
Secondly, you gain some power associated with the Chara Change. This is different for every person and every Shugo Chara. Yaya is able to pull a giant baby rattle out of thin air. Tadase is able to produce a scepter and use Holy Crown, a powerful defensive ability. Kukai becomes even more athletic. Oftentimes, this results in a weapon or ability of some kind, which varies from person to person.
A very rare few people can also Chara Transform (or literally, Chara Become). Right now, this is limited to Amu, Utau, and Ikuto, though others may gain this ability in the future. It's unknown exactly how one becomes capable of a Chara Transform, but it involves exposure to one of two magical artifacts: the Humpty Lock (owned by Amu) and the Dumpty Key (owned by Ikuto). There is only one exception to this, and it will remain the exception, rather than the rule.
So what happens when your Heart Egg becomes darkened? When you lose confidence in your potential, when you think that you'll never be able to reach your dream, the Heart Egg turns black and becomes an X-Egg. Oftentimes, an X-Egg will leave the host and can cause trouble... the more X-Eggs around, the more trouble, and they tend to attract one another. If an X-Egg is destroyed, it's gone for good, and the owner will never be able to accomplish their dream. However, it is possible to purify an X-Egg, returning it to its normal state, after which it will return to the owner's heart.
In some cases, an X-Egg may hatch into an X-Chara, which is much more destructive and dangerous.
So where's the conflict? Well, you see, there's this legend. Somewhere out there is one special Heart Egg, called the Embryo. It's said that whoever controls the Embryo is given a single wish, anything they want. Little else is known about the Embryo, save that it's pure shining white.
A few years ago, the Easter Corporation, under the new leadership of the mysterious Gozen, decided that it wanted the Embryo, at all cost. Of course, when that cost is darkening the heart Eggs of innocent children, the Guardians will always stand in their way.
This theme has a lot of potential for OCs. While people with active Shugo Charas are usually limited to kids, there are those as old as 17 who have one. Most of the cast is in elementary school, but both Utau and Ikuto are teenagers. Adults with Shugo Charas are almost unheard of, since most of them have either long since given up on their dream, or they've gotten well underway toward their dream and have no need for a would-be self. OCs with Shugo Charas will only have access to Chara Change at the beginning, but who knows what might happen if they encounter Amu or Ikuto at the right time?
Easter is known to hire both Shugo Chara users and gifted researches and scientists. Most of the employees are able to see Shugo Charas, whether they have one or not. Those that don't are often good at manipulating children's emotions to encourage them to give up their dream. After all, if the Embryo is a Heart Egg, and an X-Egg leaves the host's body, the chances to find the Embryo improve if they darken as many hearts and collect as many X-Eggs as possible.
The Guardians have a fixed set of positions, but anyone can be an ally to them. They are aligned with the Magic Association and also help to police Ohtori of any sort of danger, not just X-Eggs and X-Charas. Working with the Guardians is considered a high honor. Just look at how cool those capes are! OCs with Shugo Chara can be in any group and any school, however.