She is Maid Marian. She is Penelope. She is Princess Buttercup.
The One True Love is invariably a woman. The nearest archetypal equivalent is Prince Charming.
The One True Love is virginal. Only the Hero can consummate her love.
She may be a low-born woman but she can never, ever be a Fallen Woman.
She does not crave sex. She is more than happy to provide sex to the Hero (once he has Proven Himself and only within the Proper Context), but sex isn’t her goal.
She does not have sex on-screen (or on the page). It is assumed that she has orgasms but that is not to be spoken of.
She is decidedly not kinky. It can be assumed that the One True Love prefers the One True Position.
Although she usually experiences Love at First Sight upon seeing the Hero, her love must, never the less, be won through adversity and peril.
She is vulnerable. She can be killed. If she is killed, she will be avenged. Avenging her will always be the Hero’s first priority.
The Hero will always be faithful to her (in heart if not in body) unless she Lapses and becomes a Sinful Woman, in which case he will have the option of finding his Other One True Love.
The Villain will often crave her. If he does, his goal will be to Ravish her. If the Villain succeeds, the ravishment will be fatal so that the Hero can avenge her. No One True Love ever survives being ravished.
If she survives and the Hero Puts Down his Sword, she will birth him at least one son. This is an implicit part of living Happily Ever After.
The One True Love is never barren.
If she has children, she will become a Good Mother. This does not negate her status as the One True Love.
The Villain may imprison her. If this happens, she will need to be Rescued. Only the Hero can Rescue her. He must do so before she is ravished.
If she falls ill – usually because she has been cursed or poisoned – the Hero will need to take up a Quest to Find a Remedy. She will always be cured of her illness, but only in the Nick of Time.
If the hero has Loved Another, she will fulfill the role of the Other One True Love. She will not question the Hero’s love for her, nor attempted to denigrate his previous love. Regardless, she, herself, may never take another lover without become a Sinful Woman, even if the Hero chooses to leave her.
The Hero's love for The Other One True Love will never be as deep as for the One True Love, but it will be strong enough.
Her only defense against harm is her virtue. If she remains virtuous, she may be protected from Evil (or may not, in which case the Hero has to avenge her); however, if her virtue lapses, she is invariably doomed to suffer and probably die.
If her virtue lapses, her only hope is to Repent of Her Sinfulness. If she repents, she may be saved but she will never again be the One True Love.
She believes in God and is a godly woman.