The Hero of Men is the name associated with a great hero spoken of in the legends of the Picori Blade and the mythical Light Force in the land of Hyrule during the Force Era. He was the first owner of those two special gifts given to him by the Minish to assist him in his efforts to battle the many evil creatures plaguing the land of Hyrule long in its past.
Long, long ago in the land of Hyrule, during its early development after the Era of Prosperity, the day came when many powerful beasts suddenly appeared and began to lay siege to the kingdom. Just when Hyrule was in the verge of destruction, the Picori came from the sky and gave the Hero of Men a magical sword and a golden light. Using these tools, the Hero of Men successfully defeated the monsters and drove darkness away from Hyrule. The young hero then sealed the enchanted chest with the Picori Blade itself, locking all the realm's evil away within the confines of the chest and bringing calm to the kingdom. This young man became a celebrated and revered hero amongst Hyrule's populace, and his status became one that would be immortalized in Hylian lore, eventually earning him the title of the "Hero of Men".
The true identity of the Hero of Men remains a mystery to the present day, as his identity was never officially confirmed within The Minish Cap or by Nintendo or associates. There have been theories to suggest his possible identity however listed below, each having evidence to support them but none of which has yet been confirmed by Nintendo authorities.
King Gustaf was an ancient King of Hyrule countless ages before the events of The Minish Cap. His spirit returns during the events of The Minish Cap to provide Link with the Kinstone Piece that is necessary to access the Cloud Tops. Both of the Hero's tools used in the conflict were in one way or another in the possession of the Royal Family of Hyrule (the Picori Blade stuck in the Bound Chest and the Light Force residing in Hyrule's princess); Seeing that the Light Force originally belonged to the Hero of Men and was passed along the line of the Princesses of Hyrule thereafter, it would make sense that the Light Force got into the Princess' line through someone related to them.
Swiftblade the First
Swiftblade the First was an ancient swordsman who appears as a ghost to train Link in the arts of the sword after learning seven new moves from the other Blade Brothers in The Minish Cap. Swiftblade also was alive in the past as was Gustaf, although there is no indicator of how long ago he lived, contrary to Gustaf who described his lifetime as taking place "countless ages ago." There doesn't seem to be as much evidence to support Swiftblade the First being the Hero of Men when compared to King Gustaf (who plays a more instrumental role in the story of The Minish Cap by helping Link advance in his quest for the Wind Element), although he remains a possible candidate for good reason.
Swiftblade the First did live in the past, although gamers are given no indicator of just how long ago it really was that he lived, making it difficult to determine if he was alive at the time of the conflict. The title "The First" indicates that he is the ancestor of the current Swiftblade Brothers, but there is no indication in-game of how long ago he lived.
He is confirmed to be a highly skilled swordsman that lived in the past, starting the tradition of the Blade Brothers.
When he teaches Link the Great Spin Attack, he refers to it as an attack that "Only one who holds the Sacred Blade can perform." This may indicate that he has wielded the Picori Blade before.
A Separate Incarnation of Link
It could also be that the Hero of Men is just some past version of Link that has yet to be revealed in a Zelda game to date. Evidence for this stems largely from the Hero's appearance in the stained glass windows largely mirroring Link's classical appearance: short blond hair, a green tunic, and even similar facial features to the Link featured in The Minish Cap. The prophecy that states that a hero named Link will arise to defeat evil in Hyrule is also consistent with this interpretation.
The Hero of Men in the introduction.
↑ "A long, long time ago...when the world was on the verge of being swallowed by shadow..." — Introduction (The Minish Cap)
↑ "The tiny Picori appeared from the sky, bringing the hero of men a sword and a golden light." — Introduction (The Minish Cap)
↑ "With wisdom and courage, the hero drove out the darkness." — Introduction (The Minish Cap)
↑ "This is a technique that only I have mastered...Only one who holds the sacred blade can perform...the Great Spin Attack!" — Swiftblade the First (The Minish Cap)