Nina Lisandrello plays the tough partner and best friend, Tess. I would argue that it is not an easy task as Tess is a hard-headed cop who needs to see to believe. But Nina performs the task effortlessly and even though she tries to slump and scowl, she is very beautiful – rivaling the title character.
Tess is a very recognizable woman, cop or not. The one who always says she will be there no matter what, but in reality means that she will be there when you do what she thinks is right. She does live up to her friendship but only after hurting Catherine immensely every time. She has a lot of double standards but not when it comes to crime. Which is also why she only accepts Vincent after she realizes that he is not really a criminal but more of a military police officer of sorts with a license to kill.
Although it is often a betrayal of her relationships, Tess is the voice of reason on the show. It is her who reminds the others (and us) that one should do ones job, follow the rules and respect the job. It is a necessary straight line when everything else is so erratic and even when she is annoyingly insistent on order, it brings a bit of relief that not everybody has lost their head.
When I say that it is not easy to play Tess, it is because she is so mono faceted. Most of the other main characters get to explore their various dark, gray, pink and white sides. And their crazy sides. But Nina has to show constant restraint to keep Tess within her very stringent story line. Plus with the added affair with her married boss, there is little appeal left in the woman.
As I mentioned under JT, I do not feel the relationship between the two. It would be far more interesting to give Tess a relationship outside of the Vincent/Catherine alliance and let her evolve through the challenges of staying loyal to both sides. Or they could let Evan come back as a new type of beast and be her beau. That should be interesting for double dating too.
If I remember correctly Nina was the only one to get any kind of good review from the critics after the pilot aired in 2012. That is an accomplishment considering how the show was otherwise shredded (and unjustly so). From what I gather from the fan base, Tess is less popular with the fans.
In all fairness, I have to say that Tess is very real too. I do not know much about NYPD or how their cops behave/work, but I know something about human behavior. So whilst she might or might not be a proper detective, I can say that Tess is very much a proper depiction of a real person. Again I have to acknowledge the writers on their character foundation.