In addition to what's been mentioned in the above write-ups, Imoen (whom I, personally, don't find annoying unlike quite a few Baldur's Gate players) is the player character's only childhood friend and the first non-player character that can join your party in both Baldur's Gate and Baldur's Gate II: Shadows of Amn.
Imoen is about the same age as the player character (around 20 years old when the first game begins). On her avatar she has pink hair, though her character portrait's hair in the first game is more a shade of red (and not a light one that could be easily confused for pink). Her default clothing colours in the first game (which could be changed by the player at any time once she entered the party in all Baldur's Gate games) were both pink. In Baldur's Gate II, her default colours were black and yellow. I've been unable to find out which voice actor plays her part for the games at this time.
Both Imoen and the player character were orphans raised by a Harper and powerful spellcaster, Gorion, in the library fortress of Candlekeep (a place where children are very rare) in the Dungeons & Dragons world of the Forgotten Realms. They share another significant similarity as well...
Warning: The following text contains spoilers about the storylie to Baldur's Gate II: Shadows of Amn.
Both the player character and Imoen are Bhaalspawn: They are both the result of Bhaal, the dead god of murder, fathering as many children as he could with various species throughout the Realms after forseeing his own death coming during the Time of Troubles. Due to Imoen's light-hearted and good nature, the evil pull of Bhaal within all Bhaalspawn (that fights internally with the player character throughout the games and turns many Bhaalspawn chaotic evil) doesn't affect her until Jon Irenicus brings it out by creating a dark place in her mind through torturing her and showing her murder. Imoen is the player character's half-sister, though neither of them are aware of this until about halfway through Baldur's Gate II.