For the scope of the question (within video games), Zelda is the noted princess of Hyrule for game series The Legend of Zelda.
Daughter of King Rhoam Bosphoramus Hyrule, Zelda is commonly noted to possess the Triforce of Wisdom & a descendant of the Great Sages through the various games in the series.
CAUTION: SPOILERS AHEAD FOR A COUPLE OF GAMES!
Zelda has been noted to disguise herself in various games in the series to avoid being discovered by her enemies. Within Ocarina of Time, she was disguised as Sheik to avoid detection from Ganondorf. In Wind Waker, she was completely disguised as the pirate captain Tetra (who was later revealed midway in the game).
In most games, Zelda's role in the series has commonly been the typical "damsel in distress" at one point or another in each game. Sometimes being the main plot at the beginning while others utilized it later in the game, but always been providing assistance to you in some form.
Please check out the games within the series for more details about her role within those titles.