So, who is this guy, exactly?

Well, he's a pretty underdeveloped bad guy in Past Sins. We saw a lot of him, but he was not really himself during the entire story.

Genius enough to create Nyx with a complex multi-phase spell.
Intelligent enough to be the headmaster of Celestia's elite magic school.
Insightful enough to put preventative measures on himself just in case the remnants of Nighmare Moon turned out to be SuperBad (which they did).

When he's under the influence of the Evil Entity, he does some blindingly stupid things of course. Most of his time in Past Sins is under this influence, so we don't really get to see what Nexus is like normally.

So the question now becomes, who is he during my story? I get to write him however I want, so this thread is to talk about Nexus and try to flesh out his character a bit.

In this first post, I'll try and assign Nexus a personality type as a place to start.

Above all, I think Nexus needs to retain his intelligence, because let's face it—you don't create resurrection spells and head the most elite school in the nation unless you're damn smart. But being "smart" does not a personality make. What kind of smart is he? How does he process information? What influences his interpretation of the world?

To begin with, I suppose I should assign a personality type to him. So let's take a look at the choices.

As a smarty-pants type, it's probably safe to say that Thinking will be in his top two functions. Is he introverted or extroverted thinking though?

He's also a theorist, which is an intuitive trait. Intuition is also likely in his top two functions. This puts him in the NT quadrant. Also known as the Rationals.

For Sensing and Feeling, those are further details we can fill in to make the picture complete. For now, let's figure out what kind of NT he might be.

There are two very different flavors of NT personality. There is the introverted thinking which is a vast library of facts and insatiable quest for knowledge. This is coupled with extroverted intuition, which is an idea generator and tends to make one a scatterbrained inventor type. These are the NTP types. Like Twilight.

The opposite flavor of NT is with extroverted thinking, which is more of a pragmatic strategic thinker that formulates elaborate plans and schemes to accomplish a specific focused goal. This is coupled with introverted intuition which is the primary driver behind deductive logic. These are the NTJ types. Like me.

As for introverted or extroverted, I think it's safe to say Nexus is introverted. The extroverted version of this would be someone like Trixie.

With these clues, I'm leaning toward assigning Spell Nexus my own personality type of INTJ, which I happen to know quite a bit about, since I am one.

How does this affect Nexus? Well, it is what makes him able to focus endlessly on complex details to derive a working solution to a singular near-obsessive goal. Kind of like the effort of writing a lengthy Past Sins / Fallout Equestria crossover. Crazy stuff like that. I see this coming through in Nexus' Megaspell project. He has not let go of his dream of finding application in the spell which Discord taught him years ago. And he is determined to do something good in this world to make up for his failure with the Nightmare Moon incident (despite having been forgiven for it).

So I could speak endlessly about the traits of the INTJ which apply to Nexus. A few of them would be things like his social difficulty. INTJs are not known for their social prowess. This is not to say that Nexus is impolite. His personal circumstances outside his personality have given him a fear based response to authority. Look at that first image at the top of this thread. Nexus has a fear/respect relationship to ponies in authority. He prefers to discuss things from a scientific perspective, but he is easily intimidated. This could cause him to suddenly lose his confidence even if he's right, simply because he is challenged by someone who appears to have done all their own research.

Nexus is, above all, intuitive. He "sees things" that he cannot easily explain. One of the INTJs weaknesses is our difficulty communicating internal intuitive ideas to others. A classic example of this is Nikola Tesla, who had some fantastic ideas... which died with him, because he couldn't communicate them. Sigh. Anyway... Nexus is going to understand things before he understands that he understands them. That's introverted intuition. It also is why he works alone, because having a team of researchers would actually slow him down because he would have to micromanage them everytime someone got off on a tangent that didn't fit his internal vision - and micromanaging people is super-draining for an INTJ.

Nexus' extroverted thinking is that part of him which is driven to action. It's not his primary trait but it gives him the fuel he needs to finish the job. He is also likely to have significant confidence and perhaps a bit of arrogance about his ideas. This will be suppressed in the presence of his superiors, or anyone who he finds intimidating. One thing INTJs are classically known for is being "the man behind the curtain" so to speak. The guy who is really in charge, but not the one in the spotlight. INTJs hate the spotlight. We want to implement our plans via someone else who is more charismatic or a figurehead type person. This actually fits Nexus' behaviors in Past Sins quite well. Nexus himself wanted to be in a position of power by putting Nyx in front of him but with Nexus secretly orchestrating things. That's an INTJ gone bad, to be a megalomaniac "Brain" takeover the world type.

Third in line is Nexus' introverted feeling side. This means he's very in touch with his own feelings or the feelings of anyone else who has gone through the same trials he himself has experienced. Sympathy, basically. Not empathy. Nexus is going to be harboring tremendous guilt over his actions from Past Sins. It will tear him up inside, but that doesn't necessarily mean that he really understands what others are feeling. He's not like Rarity or Cheerilee who clearly are sensitive about what others are feeling. But Nexus will have a deep well of internal feelings that usually remains hidden from others. And he will try to find a project by which he can do some good that will make him feel better about himself. In this case, the Megaspell healing project.

No real need to dive into the nuances of extroverted sensing and the need for a little excitement once in a while. It does create a need to "show off" to a small degree, and Nexus is likely to be very proud of his Megaspell project and want to see other ponies getting some benefit from it, and maybe putting his name out there just so he can feel good about having made something to be proud of. Not for fame - INTJs hate fame. But it's just a small need to be able to show others that we can do cool stuff too.

Relationships... Well, going with the theme above, I'd say that Nexus is likely to have one or two "close" friends, and not really many (if any) "casual" friends. He is likely to compartmentalize his social circle into professional or personal categories. That means he's unlikely to be friends with his coworkers outside of his job. His boss, is very easy going though, which sets Nexus at ease and gives him the flexibility to create without being hindered by external rules and protocols. Folks like Nexus or myself have a very difficult time working under type bosses. Very.

So Flutters has given Nexus his "dream job", really. He doesn't have to make sure she understands everything. She's the artist and architect to the Ministry of Peace and its projects, and Nexus has full license to do with as he pleases within his own department.

Luna on the other hand, intimidates the crap out of Nexus. She's the ruler of the nation, and he is naturally intimidated by that despite the fact that Nexus himself does not really care for protocols and hierarchy. He simply knows (pragmatically) that Luna could wreck his world, and he doesn't want to rock that particular boat. So whenever Nexus deals with Luna, he is going to be 75% obsequious and 25% arrogant scientist know-it-all. This comes though pretty well in the dialog which they have in regards to the Amplification Spell. Nexus is confident from a scientific perspective, that he has knowledge beyond Luna's understanding. But when Luna (who is also not particularly sensitive to the feelings of others) gets confrontational about it with him, there is a breaking point where he cracks. And that breaking point is when he has to admit where he got his new knowledge from. You see, the knowledge itself gives Nexus his confidence in this discussion. But he is not thinking about Luna's reaction to where this knowledge came from. Then, when it becomes clear to him that he will have to cough up that information, he knows full well that Luna could lose it and shut him down (if she wanted) and he is genuinely nervous about it.

That's just a sneak peek into the mind of an INTJ when dealing with authority figures. I've had to deal with this throughout my career when trying to convince a superior that my idea is worth pursuing.

What about Twlight? Well, INTPs make the best bosses for an INTJ. They are great at giving ideas to "un-stuck" the INTJ who has found himself in a rut. Unfortunately Twilight isn't Nexus' boss. But if she were, I would imagine the two of them working quite closely on the same project. Since Twilight in this story is pretty busy managing her ministry, I don't think she has a lot of contact with Nexus. From what she does have, I imagine them getting along quite well.

And Nyx? It's very hard to say. I don't imagine they see much of each other. In the rare cases where they are together, I imagine Nexus has a fatherly attitude toward Nyx.

I think Nexus would have paternal instincts toward Nyx, being that he was responsible for her creation, and is (in action if nothing else) her father. He would probably offer advice and wisdom when appropriate. It is difficult to imagine that they have had much actual time to explore this relationship though.

So I agree with all of this in general.
except for one thing .

You say somewhere that Spell doesnt care for titles ( or something like that) I think that, slightly to the contrary , he may cling to them. Not because he is naturally elitist or snobby, but because a social ranking system may provide him a proxy, a useful guide, for Nexus to use in order to map the power relationships between him and others.

Why would he choose titles and hierarchies like this? I would argue because it is a system that is familiar to him. It appears that, apart from his own merits as a researcher and inventor, nexus was born into the Canterlot elite, probably the aristocracy. the picture of himself that hangs in his home office lists him as " sir" spell nexus. Note that this is a pony that held himself ( or at least his accomplishments) in such high regard to have a formal portrait comissioned and then hang it in his office! Chances are that this happened pre his posession, as nobody noticed that his behavior changed dramatically when he was posessed ( granted no one except twily noticed the changes in cadence when she was repacled by chrysalis but still) So this means that even if his title of nobility was not inborn but was granted to him as recognition of his service to Equestria, he is still very proud of it.

Actually let us talk about his house. Spell Nexus lives in a rather spectacular manse on prime canterlot real estate. It is true that many universities and schools give the president a mansion, but spell nexus's house is consistently referred to as Spell Nexus's house, rather than say, the headmaster's house. this again points to a life of wealth and privilege that predates his rise to the head of Celestia's school.
So in this way I think that Spell would be very status and class conscious. he would for example refer to Luna as " your highness" ( unless Luna specifically forbids him from doing so.) He would be very prim and proper about these things, not because he believes necessarily in some sort of aristocracy by birth, but because adhering to the mores and conventions of class and aristocracy would serve as a tonic for his social anxiety.

also Yes, Spell Nexus is arguably Nyx's" de facto" father, and i definitely feel he would be protective towards her. But I can't imagine him ever verbalizing this.

Not because he is naturally elitist or snobby, but because a social ranking system may provide him a proxy, a useful guide, for Nexus to use

Oh, absolutely! I agree completely and you have a keen and unusual insight into this personality type if I caught your comment correctly.

An INTJ does not seek a position of leadership, but will step up to the plate if no one else is competent or willing. More importantly though, they will seek leadership, if for no other reason than to allow themselves the freedom to enact their plans without hindrance from otherwise micromanaging superiors. It is a pragmatic motive for seeking titles and position. He's not caring about his own glory, but gaining a position of social, political or economic status has its benefits to help him achieve his own goals.

It appears that, apart from his own merits as a researcher and inventor, nexus was born into the Canterlot elite, probably the aristocracy

This is, of course, the second reason why a person of this personality type would care a whit about titles. It is the fact that no matter who we are, we are influenced by the way we were raised. And if Nexus perchance was raised to value prestige or titles, then he would also have a (perhaps nostalgic) affinity towards such things.

Actually let us talk about his house. Spell Nexus lives in a rather spectacular manse on prime canterlot real estate.

This is a very good point, and I think I am convinced now more than ever that Nexus is heavily influenced by such things due to his upbringing, rather than his personality. The fact of the matter is, things like "tradition, adherence to rules and regulations, bureaucracy, doing things the way they have always been done, 'just because' or placing unusually high importance on stability" are the very last thing on any INTJs mind. I speak from personal experience. Any strategist/mastermind type of person is going to be pushing boundaries and trying to determine the best way of doing something, regardless of established traditions. Unless... they value those traditions for the way they were raised.

he would for example refer to Luna as " your highness" ( unless Luna specifically forbids him from doing so.) He would be very prim and proper about these things

I think you'll like how I have Nexus speaking when he talks to Luna then.