Basic Trope: A person who keeps the world from falling apart, usually through magical means.
Straight: Himiko is a Shrine Maiden who maintains the spiritual wall separating the world from the realm of the demons.
Exaggerated: If Himiko stopped maintaining the wall for so much as a second, the world would explode from evil.
Downplayed: Himiko's prayers are more like vitamin pills than a pacemaker; her efforts make the world a better place and keeps demons at bay, but if she were to stop or die nothing catastrophic would happen.
It's the law of the realm that someone with great spiritual power take the role of the Barrier Maiden.
Inverted: Himiko is an Apocalypse Maiden— she will destroy the world simply by existing.
Subverted: Himiko seems to be the one who maintain the wall that protects the world from the demons, but when she takes a long vacation, the wall is doing okay.
Double Subverted: ...for a while, anyway.
Parodied: Himiko psychically holds the entire reality from falling into the Grand Oblivion... With duct tape!
Zig Zagged: Himiko is raised to maintain the wall keeping the demons away, but then Bob and his band wipe out the demons so she doesn't have to, but this upsets the Balance Of Good And Evil and the other world group that used to be humanity's ally turns against thm so Himiko has to make a wall against them. This buys Bob's group time to finish off the new threat, but the Dangerous Forbidden Technique they use to do so damages the world.....
Averted: The world doesn't need to be maintained with powerful magic cast by a person or a group of individuals.
Lampshaded: "This lady who stands before the gigantic pillar of light seems to be important to the world's well-being."
Invoked: According to legends, a Barrier Maiden annointed in the Temple of All-Gods shall raise a barrier between this world and the realms of the demons. Since the world is currently, you know, overrun by demons, The Hero Bob and his team wander the land to search for a girl to deliver to the Temple of All-Gods.
Exploited: Archfiend Centurios sends his demonic hit-squads to assassinate Himiko before she can become a Barrier Maiden.
Bob: "So if anything happens to you, the world will be thrown into chaos?" Himiko: "That about sums it up." Bob: "Harsh."
Viewer 1: So this Himiko keeps the world from being overrun with demons? Viewer 2: That, and it's really stressful to her, and killing her will break the barrier. Viewer 1: What kind of Jerkass God would lay that burden on one girl, and make the world be one accidental death away from oblivion?!
Deconstructed: Himiko is under unbelievable pressure from keeping the world together by herself.
Reconstructed: It's obvious that no single person can hold the world together forever, so the law of the realm is amended to make the role of Barrier Maiden rotates amongst the most spiritually powerful priestesses of the world.
Played For Laughs: Himiko dresses like a shrine maiden and regularly performs rituals she thinks holds the world togehter, and everyone who passes by thinks she's a cosplayer.
Played For Drama: Mio, Himiko's lady-in-waiting can't bear to see her beloved lady shoulder the responsibility of the world alone. That's unfair, so she elopes with her. Thus it becomes a melee-a-trois: Mio, who want to protect Himiko from the pain of being a Barrier Maiden; Bob, who want to restore Himiko to her supposed role; and Centurios, who want to assassinate Himiko so he can start his demonic invasion. Meanwhile, being outside the Temple of All-Gods allow Himiko to see how rotten humans can be, and darkness grow in her heart... she's torn between keep on running from her responsibility, returning to her role, or take the mantle of Apocalypse Maiden altogether.
Back to Barrier Maiden. Don't let her fall, or the world will come to an end.