Basic Trope: The secretive figure who appears in a moment of need, aids the hero or heroine, and then vanishes again.
Straight: In the Shounen anime Super Trope Warriors Bob, the mysterious Darkmask always appears to save the eponymous character when he's on the receiving end of thrashing from the Monster of the Week, thus allowing Bob to gather his remaining strength and deal the blow that saves the day. However, Darkmask always vanish before Bob can thank him.
Downplayed: Darkmask might appear when Bob seems to be overwhelmed, but it's clear that he's not any more powerful than Bob (and might even be overwhelmed himself if the fight goes too long).
Due to a magically-enforced Charlie Brown from Outta Town situation, Darkmask can't fight the monsters despite his sheer power, unless it's to aid another hero for a brief time.
Darkmask is really Dave, Bob's long-lost brother/close friend/etc, it's only right if the helps Bob out of tight spots. (It's a Fridge Logic why he must do so in secret and/or not fighting the monsters himself to begin with.)
Inverted: Super Trope Warriors Bob Gaiden ~ Darkmask Chapter, which is a Perspective Flip— from Darkmask's POV.
Parodied: Bob's power is too pathetic to fight the monsters, were it not for Darkmask, he would never survive any of his battles. And this is obvious to everyone but Bob himself.
Zig Zagged: ???
Averted: Bob fights alone. Always.
Enforced: "We can't have a Boring Invincible Hero... let's have him being in a pinch." "But what if he's not, logically, strong enough to break out of that situation?" "Why, just have someone more powerful than him saves him." "That's BRILLIANT! (I wanted to say that if there is someone like that then he or she should be the main character, but I don't want to lose my job.)"
Lampshaded: "Just in time, man... whoever you are."
Invoked: Bob's mentor Alice gives his friend Dave the power to turn into Darkmask, but only when Bob is in danger. Once Bob is back on his feet, Darkmask has to retreat before his powers wear off.
Defied: Emperor Evulz drag Bob into a Phantom Zone for their fight so Darkmask can't help.