GA review (see here for what the criteria are, andhere for what they are not)
Short article, but strong. There are a few issues with the prose, but not major and probably easily addressed. It's my practice to review an article against the GA criteria, and then go into more detail afterwards.
It's a "small" topic (a TV character who's in a limited number of episodes), so there probably isn't a lot of information and sources about it. I'm AGF that you've exhausted all possible sources.
Much of the prose issues in this article have to do with tenses. You do a good job at using a real-world perspective, but the tenses aren't consistent. Choose one style and stick with it throughout the article. I recommend that you maintain the present tense in the "Character biography" section, since it has an in-universe perspective, and past tense for the rest of the article, since it's real-world.
It's my practice to copy-edit as I review GAs, since I think it wastes everyone's time to direct you to make minor corrections when I can just do it myself, unless there's a point to be made.
Character biography
You should explain the kind of work the SVU does for those of us unfamiliar with the series. I happen to know from my general knowledge base that the show is part of the L&O franchise and that the officers focus on sex crimes, but you should explain that as you introduce Cassidy.
Here's an example of mixing your tenses: Cassidy has a drunken one-night stand with fellow SVU Detective Olivia Benson (Mariska Hargitay), and expressed a desire to pursue a relationship. Maintaining your tenses, it should be: "Cassidy has a drunken one-night stand with fellow SVU Detective Olivia Benson (Mariska Hargitay), and expresses a desire to pursue a relationship with her."
Choppiness is also an issue with this article. I wonder if you could re-structure these sentences, to read: "Cassidy has a drunken one-night stand with fellow SVU Detective Olivia Benson (Mariska Hargitay). He expresses a desire to pursue a relationship with her, but she turns him down, citing a policy of not having relationships with coworkers."
Years later: Please be more specific; state when she has her regrets. (I assume it's when he returns 13 years later.)
This and the stress of the unit...: "This" could be anything that goes before it. I assume you're talking about his unresolved relationship with Benson; if so, please say so.
Gang-raped: It's my personal preference to use a pipe to make sure the link works; I changed it, but it's up to you if you want to continue the practice in other articles. (I think it looks better in edit mode.)
...Cassidy decides he lacks the stomach to deal with sex crimes: This isn't the best encyclopedic language. I recommend changing it to more formal language, which I'll leave to you since you're familiar with the series and the character. How about: "...Cassidy decides that he is unable to deal with sex crimes"? You could also directly quote Cassidy, since you're using the actual episode as a ref, such as: "...Cassidy decides, as he puts it, that he "lacks the stomach" to deal with sex crimes".
Munch later laments: I have issue with the word "lament"; again, it's unencyclopedic. Change it to a more formal word that fits the character, such as "complains" or even "states".
You should state somewhere in the first paragraph that everything described occurs in the first season, or you need to state that he "returns" (I thought that was a better choice of words) 13 years later in the second paragraph. Since you're using in-universe, you could remove the reference to the episode he returns, since all that information is in the infobox, and state that he returns 13 years later.
He has been working undercover for the past three years as a bodyguard for a pimp, Bart Ganzel (Peter Jacobson), his former colleagues at the SVU are investigating, and provides the detectives with information about Ganzel's prostitution ring. This sentence is unclear. How about: "He has been working undercover for the past three years as a bodyguard for Bart Ganzel (Peter Jacobson), apimp his former colleagues at the SVU are investigating, and provides the detectives with information about Ganzel's prostitution ring."
Last sentence: What happens after the kiss?! Are Benson and Cassidy together after that? Or does the series leave us hanging about the status of their relationship? Either way, you need to tell us. Perhaps we need an update if this was addressed in subsequent episodes.
Development
where Belzer was a regular... I have a procedural (har-har, pun intended) question. Is it customary, when talking about TV series, to state this in this way? It makes it seems like the show is a place, which I know is often the colloquial usage--you state, "He was on L&O for two years". If so, we can leave this as is. If not, we should change it to something like: "on which Belzer was a regular".
he credits fellow cast member Richard Belzer... Another tense agreement problem. I went ahead and fixed it to past tense, and I'll fix other errors.
Too many hads, as in "he had previously worked with Belzer", and too many alreadys which I also removed because it double-modifies the subsequent verbs.
Winters had a role at the same time on the HBO drama Oz and, while the SVU role was initially only supposed to last a few episodes, he was contractually obligated to Oz, and eventually departed SVU completely to focus on Oz. This is a little confusing. It seems like you're saying that he left SVU because of his contract with Oz, but his role was designed for only a few episodes because of the conflict. Which is it? Did Winters want to do both shows at the same time? I see from the Green book that it's unclear. I'd say something like: "Winters was contractually obligated to HBO due to his role at the same time on its drama Oz, so he had to leave SVU after a few episodes".
Winters believed that Cassidy was not unintelligent but just a little naive. He voiced early on a desire to executive producer Ted Kotcheff that Cassidy not be made into the dumb blonde of the unit because he did not believe there would be any in the Special Victims Unit. I see that these are Winters' words, quoted in Green. They're encyclopedic phrases, so I suggest that you quote Winters directly.
grizzled: Another unencyclopedic word; I suggest quoting Winters directly again, especially if that's how he put it in the interview, which I won't spend the time checking.
Reception
Variety's: You don't need to put the possessive ('s) on the outside of the pipe in this instance, since the link will work. Another picky point, but I bring it to your attention because you may not be aware of it.
Punctuation should be on the outside of quotes, as per MOS:LQ. I changed it.
Entertainment Weekly's Bruce Fretts agreed that we do not see nearly enough of Cassidy. Unencyclopedic phrasing; I see that Fretts uses this exact wording, so either quote him directly or paraphrase to something like: "...Fretts agreed that Cassidy does not appear as much as he would have liked."
Sources
Green (I was able to access it on Google books): Some of the statements are cited incorrectly. For example, the 2nd sentence in the 1st paragraph in the "Development" section is actually on p. 157, not p. 156. I suggest that you go through the Green refs and correct them.
Ref12: You don't need to include the subtitles from articles in popular sources, since they're really meant to describe the article, and not as part of the actual title.
b(citations to reliable sources): 
