Queen (Japanese: クィーン, 女王, Queen) is the name of several separate classes occurring in the Fire Emblem series. Each one occupies a position of royalty, and is exclusive to one character.
In Genealogy of the Holy War, the Queen class is a personal class that belongs to Hilda, a major antagonist in the storyline of the second generation. She uses fire, thunder, and wind magic, as well as staves, and appears in the tenth and eleventh chapters. Being an endgame boss, she has high stats with a variety of weapon choices.
In The Sacred Stones, it is a class exclusive to Ismaire as an NPC, the queen of Jehanna. The class is not playable, it only appears in cutscenes, and as such, the stats the class has are irrelevant. When Ismaire becomes playable in the Creature Campaign, she instead uses the Swordmaster class, though the queen class she has earlier can also use swords, this is never demonstrated.
The latest appearance of the class is in Radiant Dawn, as the personal class of Elincia, crowned queen after the events of Path of Radiance, in which she used the Princess class. She wields swords and staves, and flies on a pegasus, which gives her the Canto skill; additionally, it is counted as a third-tier class, with the special skill Stun. Generally Elincia as a queen is a fairly strong fighter, but surely not excellent in combat.
The flexibility of the "Queen" name lends itself not to its role in gameplay but its role in storylines. Fire Emblem, being a fantasy series, often includes the conflicts of warring kingdoms, most of which are monarchies; as such, the existence of the Queen class is often necessary for narrative scenes and battle.
In the coding of the English release of Fire Emblem, there are two unused classes known as "Queen." Both are unused, though it later appears in Fire Emblem: The Sacred Stones as Ismaire's NPC class, in Fire Emblem it presumably would have belonged to Hellene or Eleanora, though Hellene never appears on a map, and Eleanora instead is a Bishop. The other queen is mistranslated, being "Princess" in the Japanese version, that most likely would have been used by Guinivere.