Valentine isn't real.
- Look at how ceaselessly supportive she is of her employer, how she continually encourages her to let go of the past stardom and career-making role by moving forward and embracing the role that Maria's colleague once inhabited and making it her own, and how her efforts are continually confounded by Maria's refusal to let go; she also has little to nothing of a life outside of her work, and her support of younger actresses more notorious for their lifestyles but having hidden depths could easily be a mixture of Maria's own mixed support and dubiousness surrounding said actresses. Valentine also doesn't even have a last name; she's simply part of Maria's own mind, the part coaxing her away from clutching to the past too much, who eventually gives up the ghost when she realises there is no hope for Maria.
- Ultimately, hard to say: at one point, Jo-Ann comments that she's talked to Maria's assistant, at a time when Valentine is still around, suggesting that Valentine does indeed have some kind of independent existence. Still, the fact that she simply disappears from the film, and Maria appears to entirely forget about her, does support the idea that she's not quite real.