Basic Trope: Mysterious young female who is pivotal to the plot.
Straight: Alice is an ethereal, fascinating young woman who meets Bob, who agrees to protect her and her lucky amulet in exchange for the use of her healing powers.
Exaggerated: Aerith is a completely cryptic and inscrutable young woman who is really seven hundred years old, has vast magical powers, is self sacrificing almost to the point of being a doormat, and her locket holds the key to utopia.
Downplayed: Aerith isn't that cryptic and she looks like the Girl Next Door aside from her unusually dark eyes, but she can't give a satisfying explanation of where her amulet came from.
Justified: Having to hide from the big bad, or just from an misunderstanding society at large, because of her gifts, leads to a certain sense of 'distance' from everyone else.
There is nothing mysterious about the woman at all.
Subverted: Bob comes across Alice, who initially is a bit distant and ethereal seeming, but she was just distracted when he showed up. Her necklace turns out to simply be a gift from a girlfriend.
Double Subverted: However, that is just a ruse, she has even greater secrets than initially imagined.
Parodied: Bob, wanting to be a great hero, goes to find any sort of mysterious girl, convinced that it'll set him on the path to defeating the Big Bad, which is simply his jerkass history teacher who happens to have white hair.
Zig Zagged: Alice has a very special, ornate necklace...which is simply a gift from her best girlfriend...which said girlfriend stole from her uncle's collection of ancient artifacts...but the ancients make useless bric-a-brac too...however, after wearing it, Alice's healing powers surface...
Averted: There are no mysterious waifs anywhere in the setting.
Enforced: "A mysterious woman with magical powers sought after by the Big Bad will be the perfect way to make the Hero jump at the call."
Lampshaded: "Are you the mysterious girl everyone is talking about? Could you please heal my wound?"
Invoked: Alice learns healing magic and attains a powerful magical charm greatly sought after by the Big Bad, knowing that a hero will come to her aid.
Defied: Alice makes a special effort to be as unmysterious and normal as she can, perhaps even rejecting powers and items if she feels it would draw attention from the Big Bad.
Discussed: "This mysterious girl needs our help to save the world from the Big Bad."
Conversed: "Hmm, magical powers, hiding a magical crystal, air of mystery...the Big Bad will probably be after her."
Deconstructed: The mysterious waif's tendency towards self sacrifice is easily exploited by the Big Bad, thus making quick work of her, and the party ends up grieving.
Reconstructed: However, her magical powers allow her to return to life, or at least a guaranteed ascension to a higher plane.