Lore:
Magical Girls: What better way to fight negativity than with one's own positive attributes? Offered such a chance for a trait they exemplify, said abstraction determines their transformed appearance and the powers they gain. Even in civilian form, they keep certain advantages - for example, it's nigh impossible for one to die by mundane means. She may be injured in a car crash, but less so than an ordinary human, and would survive the ordeal. The only real threats are other beings of a magical nature.
Keepers: Small fluffy carbuncles, visible only to people they want to be seen by, namely those they seek out for their potential as magical girls. After granting power to these chosen few, the Keepers serve as loyal guides. Varying in behaviour, as well as fur and forehead gem colour, these beings nonetheless have a common goal: Recruit warriors of light to cleanse away the darkness. Of course, there may be more to this than they let on.
Vices: Monsters born from dark thoughts and feelings, be it despair, envy, hate, or anything else that can consume a person with negativity. Invisible to regular humans under most circumstances, they can nonetheless be sensed - a sudden shiver of dread, or that unexplained feeling of being watched, can indicate the presence of a Vice. Appearing in many forms, these creatures affect their targets in a variety of ways - draining physical health or positive emotion, exacerbating a person's harmful tendencies, sometimes even attacking directly when they reach a certain level of strength. Unharmed by conventional methods, the only way they can be destroyed is through magic.
Mana Crystals: A gem created when a girl first makes a deal with a Keeper, allowing her to transform into her magical self. Taking the form of a simple accessory, the crystal is surprisingly durable, remaining intact even in the heat of battle. If taken more than twenty feet away from its owner, it will simply vanish and reappear on her, so there's no need to worry about the loss of such a precious trinket. Once a magical girl, always a magical girl.
Mana Shards: When a Vice is defeated, a fragment of crystallised residual magic remains. This can be absorbed into a mana crystal to boost power and take on stronger enemies. The idea of harnessing dark energy may unsettle some, but the keepers are quick to quell such concerns. The virtuous base power within a mana crystal will cleanse the taint from a shard - at least, at first. Possible effects of continuous buildup on the mind and body are something new recruits have yet to discover...