For some reason I have been thinking quite deeply about Dexter's actual type. At first I was willing to agree with the general consensus that Dexter is an ISTJ, but upon further re-evaluation of his character development over the course of the series, I actually think there are strong implications pointing Dexter towards being an INTP. I'll elaborate on my reasons why:
1. The signs of inferior Fe
This is perhaps the trait that struck me the most. Dexter is extremely socially awkward and this is a fact he admits many times throughout the series. He often expounds upon his inability to connect with people, and constantly talks about how he's empty and seem to be missing something in inner monologues, which is always in relation to his inability to create successful interpersonal relationships. For instance, there is this particular scene in season one when Dexter is seeing Rita and a very socially awkward moment occurs as Dexter tells Rita that what he wants is a normal life. He is not interested in great adventures, fame and fortune. He simply wants to feel socially connected to people like he perceives everyone else to be.
This seems to point towards Dexter not being particularly materially oriented which is not very much in line with ISTJs. Being auxiliary Te, ISTJs do seek out material wealth, especially in order to create a strong stable life for themselves. This is because Te as an auxiliary function makes the ISTJ measure personal success in the material world, due to the very nature of Te itself. Te can only compare impersonal facts externally, so to the xSTJ, one way to for example measure personal success is to have the most expensive TV available on the market. This is not the kind of life Dexter seems to express that he wants to have. He seems to in fact be very idea-oriented and he is very focused on the idea of what a "normal life" is.
The struggle between what Dexter perceives himself to be and how this is incompatible with societal standards seems to strongly points towards the fact that Dexter experiences inferior Fe. He is extremely sensitive and aware of what others think about him and may in fact quickly change his behavior and attitudes in order to comply to group think if he senses that it will lead to social acceptance. He also often expounds upon what he thinks to be "normal" behavior and what it means to be "normal", and then later subsets this behavior with how he is not. Yet again, he is showing a strong willingness to adhere to what he thinks is "normal behavior" in order to pass himself off as normal, which he has deeply internally rationalized as an extreme form of survival. That kind of think also seems to be very much Ti rationalizing why Fe is needed at all, and I definitely recognize myself in Dexter's struggles to gain social acceptance. I also want to compare Dexter to the character Temperance Brennan from the TV series Bones who seems to clearly exhibit IxTJ behavior. However, in contrast to Dexter, Brennan is actually not particularly willing or interested in adhering to group think in order to get along, but instead seems to have a fiercely strong conviction to be who she is, pointing towards her tertiary Fi. There is little to no Fi in Dexter. He seems to almost have no sense of identity and the identity that he has is entirely built around how he is not normal.
2. The SiTi Loop
I think the real culprit why people mistake Dexter for an ISTJ is because Dexter has a strong and well-developed Si to the point where he appears as an Si-dominant. Dexter does indeed seem to spend much of his time in the past. He constantly refers to the past and he usually relies on his past experiences both while at work and elsewhere. However, Dexter lacks the stability of an Si-dominant. Everyone can agree on that Dexter is mentally unstable, even if we disregard the fact that he's actually a serial killer. Yet Dexter has a complicated relationship to his past. In the first season we see Dexter struggling to repress his memories when he saw his mother die. He also has a general attitude of wanting to move forward in life and leave the past behind him. He wants and desires change, and these various facets of change are dealt with in every season, again especially in relation to develop interpersonal relationships. Season three focused entirely on Dexter's craving to have a good friend with whom he could share who he is with, and we saw a similar thing happen in season five where Dexter again hoped that he had finally found that someone he could share everything with.
The overall impression seems to be that Dexter is not stuck in the past because he desires to live in the past, but because he does not seem to know how to leave the past behind him. We don't see Dexter ever adhering to SJ-values just because. To Dexter, social values must always be rationalized first, he has to understand why they are important. It for instance occasionally happens that other characters in the series lecture him about this, especially Debra (ENFP). She may lash out at him in anger and his seemingly apathetic behavior asking him to care a little bit more.
This is diametrically opposed of the nature of a strong Si-dominant who use their auxiliary Te in order to reinforce the SJ-values to create stability. Yet Dexter is far from a stable person, and this juggle between stability and instability (read spontaneity) is expressed in his apathetic behavior that is very much in line with the SiTi loop. Dexter constantly struggles with the feelings of why he should care about those around him. Would it not be because Dexter is enneagram 4w3 but instead say, enneagram 5w6 which is more common for the INTP, he would probably not even bother try to keep up his social image as he would struggle too much with finding a reason why he should care doing so.
Other things that point towards Dexter actually not having inferior Ne is that he's highly adaptable to new situations. He does not become stressed when the world around him becomes unstable like an SJ would. If anything, he seems to thrive on this instability which irritated the character sergeant Doakes to no end in season two (ESTJ). He has no problems coming up with solutions to get away in the last minute and Dexter has almost an over-emphasis on certainty. He must constantly rule out every possibility that his victims may in fact be guilty for example. The way he also always comes and goes as he pleases at his job does not imply that he is an Si-dominant. Dexter wants and desires flexibility so he can juggle his dual lives. Compare how this eventually clashed with Rita's (ISFJ) desires to create a stable family life.
3. Enneagram numbers can be confusing
I think the real culprit why people think Dexter is an ISTJ is because he seems to project ISTJ-like behavior. He is for example extremely pedantic (1w9) and surprisingly enough not a slob which is otherwise the norm for the NP. He also works as a blood spatter analyst which can be considered to be an ISTJ profession (5w6), but it's important to understand that this is all a projected image on his part (4w3). This need for projection stems from his childhood and is probably reinforced by Harry's codes of how Dexter should live a life to not get caught. Indeed, stereotypically enough, the ISTJ is often described as the model citizen. In Dexter's mind, if he can come off as an ISTJ, he can finally find the social acceptance he seeks. His greatest fears therefore seem to be congruent with the core of enneagram 4w3 with a very strong 3 wing. 3 because the 3 is an image enneagram and is also more commonly associated with Te-users, but when looking further beyond the more superficial image projection of 3, he's envious. The vice of 4 is always strongly presented when he keeps talking about how he cannot be like others. He is essentially doing typical 4-esque behavior by defining who he is by what he is not. His tritype seems to be 1w9, 4w3 and 5w6. I would also argue that he's instinctual stacking is sp/sx.
I also want to stress that Dexter is really Ti-dominant. I understand why this can be confused with Te to some since he also references to external models, but if he didn't, he would not be a particularly good blood spatter analyst. However, when Dexter arrives at a crime scene he always seem to rely on what he knows (Si) while running different scenario outcomes (Ne) to reach very quick conclusions about what happened. Dexter is very fast at judging, and fast judgement speaks for a dominant internal judging function. The real difference between ITPs and ITJs is that ITJs need to collect data in order to justify their internal models, meaning that externally, ITJs can appear as slower to arrive at a conclusion than the ITP. The ITJ will always try to find data that justify their models regardless of how much time it takes for them to find that final piece of data, whereas the ITP will simply run a couple of scenarios of what happened in their head and judges on what they think is the most likely conclusion of what happened. When the INTP has a strong and well-developed Si, this judgement can go extremely fast. They can essentially simply look at a new situation and say exactly what happened and how it happened without even seemingly paying much attention to Te details at all as Ne runs through various Si models, and Ti picking out the scenario that appears to be the most likely. Dexter is simply not spending much time looming over Te details in the series like we for example see the characters in Bones doing (photographs, bones etc.), but is actually content with arriving at what he thinks is the most logical conclusion of what happened. And he's usually right.
5. Some additional food for thought
Dexter has a behind-the-scenes interaction style (he prefers to stay behind and observe) which is more in line with IPs. The only other type that have this interaction style are ISFJs, but I think we can safely rule that out as Dexter is far too socially awkward to be Fe-auxiliary and he's all about the objective facts. He also has plenty of what I call Fail-Fe moments throughout the course of the series, especially when dealing with the emotions of others. Debra can for instance seek him out when upset, such as being pissed off at one of her numerous boyfriends, and have an amazing ENFP rant. When she's finished, Dexter might say something extremely socially awkward, for example "So you don't like him anymore?" (Ne + Ti) and then look extremely shocked and surprised when she in fact becomes more angry but this time at him.
He's also an informative communication style user which again is more typical for intuitive perceivers than other types, exceptions being the ESFJ, ISFJ, ESFP and ISFP and I think we can safely rule those out. Simply saying yes and no is not Dexter's type of vocabulary. Masuka might for example ask Dexter to hang out with him, at which point Dexter will often say something in the lines of : "Sorry I can't. I'm taking Rita and the kids out today." This is not very congruent with general ISTJ behavior, as the ISTJ is a chart-the-course and directive communication style type.
Furthermore, if Dexter can bend the system to his favor he will definitely do so. He is also not overly fond of authority figures although this is perhaps more obvious in the books and implicit in the TV series, and in season two his relationship to sergeant Doakes was also a typical INP versus ESTJ conflict.
I think there is plenty of evidence pointing towards that Dexter is not an ISTJ but an INTP, just not a stereotypical one. He seems to be somewhat in an SiTi spiral, suffers greatly because of his poorly developed Fe and has an usual enneagram tritype for an INTP with 4w3 as his core type and with a strong 1w9 and 5w6 sp/sx. Overall it makes him look much more like an ISTJ than an INTP, along with what is possibly a personal desire to also be like an ISTJ. He does however constantly display what is definitely Ti-dominant behavior throughout the series, and intuitive more so than a sensor because he in general seems to be more idea-oriented than focused on the her and now. His overall interaction style and communicative behavior is also much more congruent with that of an INTP than an ISTJ.
Feel free to shoot this down. I am open for critique. I am however not seeing how Dexter is actually an ISTJ as he displays all the typical characteristics of an INTP, and this is probably why I identify so strongly with his character.