This thread is far too massive to read through everyone's posts to see what they have written about DA's fare, red-haired maiden...
But I would like to give my two cents.
Leliana is easily the most interesting and well created character in the DA universe. Someone on the path of repentance with a dark, checkered past in DAO to 'Sister Nightingale' in DA2 to the spymaster of the Inquisition in DAI. BioWare must find her just as compelling of a character to include her in all 3 games in the series (and her own DLC). She is someone who is strong in her faith, yet is easily influenced by the protagonist... especially the Warden.
Obviously I romanced her in DAO and gave her the nug (in more ways than one

)... but when I was first introduced to Leliana in DAO, I didn't think much of her. As the game went on and I conversed with her at camp (and changed her out of her Chantry robes), I began to fall in love with her (in a plutonic way... as a cool videogame character). I never desecrated the Sacred Ashes, so I never killed her in any playthrough nor do I know how she came back from the dead if she was killed.
In DAI she is better than ever and her spymaster outfit with the hood is pretty awesome. Although we don't get to know her on a personal level as we did in DAO, she is still the 'cool kid' of all the advisers, and people who have played all 3 games already know her backstory.
A lot of people have complaints about DAI, but thank you BioWare for bringing Leliana back into the game, and it's good to know that she is still hopelessly in love with my Warden after all these years (even though she has know idea where he is)...