Gwen has been a mostly positive character throughout, at least as positive as anyone gets on Torchwood. She has her flaws, but she's the heart and conscience of the team.
How!? She cheats on Rhys and is a general bitch to everybody. The only way she's the heart of the show is because Russell T. Davies keeps telling the audience how "marvellous" she is. He spoonfed the audience the exact same thing about Rose. His writing is all tell and no show.