Steal from minimus and social network analysis
One of the relationship models I like the best is from Minimus. Where the player is instructed to denote a directed graph with blue and red arrows to indicate like and dislike. Every node of this graph is another person who "each person can do some-
thing useful that your character can’t; they’ll ask for something in
return." By indicating the from of the relationship and the utility of the relationship in the graph, a very simple social network can be depicted. As players play, changes in the relationship map (page 4) represent real character development.
To adopt this for a longer game, keep the same map principle and continue drawing in pen. By indicating additional likes and dislikes (also accompanied by a want/need descriptor and potentially in a subgraph if things are too fiddly) you can see easily the predominant attitude of an NPC towards anyone else (is it mostly blue? Is it mostly red? Is it thick? Is it thin?) and the historical trends of their relationship. Social Network graphs are also quite flashy. For extra credit, place nodes on index cards and connect them by yarn pinned on the wall. (One of my favourite diagramming techniques). If you play in a place that provides this, the relationship map on the wall will be a constant reminder of the game during the game session and will make relationships (and their manipulations) obvious. The ritual of adding more yarn to the wall is a tangible and real reward (or punishment) that is easily linked to feelings of real satisfaction.
In terms of linking to diplomacy checks, I would first use an alternate diplomacy system. To produce the least impact on the game, I'd borrow The Giant's alternate diplomacy rules and assign modifiers based on the graph. The count of blue to red indicates a point on intimate/nemesis and maps directly to that bonus. Risk v. reward is a case by case, with the want/need of a specific relationship providing a bonus if invoked.
This method, especially if constructed in a persistent index card/yarn mode, provides a visceral and tangible graph of relationships in the game that acts as a focal point and means of tangible reward for characters, while keeping things simple and easily searchable. A digital version of this map can be trivially made with graphviz. Beyond graphviz, I'd recommend yEd or an online flowchart tool of your choice. If you're willing to do a bit more work, the graphviz extension for mediawiki should produce excellent results. If you want a prettier layout, I recommend Illustrator or equivalent vector graphics program.
You might look around for the Best of Dragon Magazine CD set. It's unavailable on Noble Knight and pretty expensive on Amazon right now but with some searching you might find a reasonably priced copy. It has issues of Dragon from 1-250.
Also if you want more backstory on the history of D & D can I recommend Designers and Dragons? It covers not just the history of D & D but roleplaying as a whole. I've read the first edition (published by Mongoose) but it's about to re-released by Evil Hat productions (see link above). As a bonus they provide a the chapter on 1970s TSR for free.
Best Answer
Is Pathfinder D&D? No, but kind of.
Pathfinder is published by Paizo, which does not own the rights to Dungeons & Dragons. Those rights are owned by Wizards of the Coast, who currently publish D&D’s “fifth edition,” or 5e. But Pathfinder is a spin-off of Dungeons & Dragons, specifically the “v.3.5 revised edition,” or 3.5e, and is extremely similar to that game in many ways. Playing a game of “3.PF,” using material from both rulesets, is quite possible and popular.
How and why this came to be, however, requires a history lesson.
D&D 3.5e and the Open Game License
Wizards of the Coast bought TSR, the company Gary Gygax founded to publish D&D, in 1997. At the time, D&D was in its second edition, known as AD&D 2e (the “A” stood for “Advanced,” and there was also a “Basic” edition). Wizards published their own third edition (D&D 3e; Wizards dropped the Basic/Advanced thing) in 2000, and the “v.3.5 revised edition” (D&D 3.5e) in 2004.
Wizards of the Coast also released the foundation of D&D 3e, and later D&D 3.5e, under the Open Game License, which was very, well, open about how much of it could be re-used. This led to a huge explosion of third-party content for 3.5e, and 3.5e lived a rather-long life as these things go. There was a ton of material for it, the people playing it had gotten very used to dealing with, or even getting attached to, its myriad problems.
At this time, Paizo published the Dragon and Dungeon magazines under license from Wizards of the Coast. They also published a fair amount of their own adventures for the 3.5e ruleset, under the Open Game License.
D&D 4e and the non-open Game System License
Then in 2008, Wizards of the Coast released D&D 4e. The fourth edition of the game was a massive departure from previous editions of D&D, and was extremely controversial. Many players had no desire to switch to 4e, and continued playing 3.5e. Some even decided they didn’t like Wizards of the Coast’s D&D altogether, and went back to TSR’s older editions of D&D. And many did play 4e, and there are some hints that 4e did relatively well in bringing new players into the game.
So D&D had fractured its fanbase, and there were a lot of people playing D&D but not necessarily playing the edition of D&D that Wizards of the Coast was actually publishing.
At the same time, Wizards of the Coast got a lot more possessive with its property. They did not renew their Dragon and Dungeon licenses with Paizo, instead publishing those in-house, and they did not release 4e under the OGL. Instead, they created and used the Game System License, which is vastly more restrictive than the OGL was. This made it nearly impossible to develop 3rd-party content for D&D 4e.
This put Paizo into a very tight spot: with their magazine revenue taken away, the latest edition of D&D hostile to third-party content, and the edition of D&D that was their bread and butter, 3.5e, aging and slowly dying, they had a serious problem. They needed an answer, and they needed an answer fast.
Pathfinder, built on 3.5e open game content
Pathfinder was that answer. It was based on the open game content from D&D 3.5e, and pushed hard to capture the market of people who refused to play 4e and were sticking with 3.5e. By promising 3.5e-but-better, and by delivering fresh content, Paizo could keep 3.5e alive, and therefore continue to make adventure material and maintain that revenue stream.
This worked. Through a phenomenal hype machine, Paizo could offer a game system that amounted to a few houserules applied to 3.5e, call it “better,” and capture a pretty large market share. It cost them relatively little to do it, and perhaps more importantly, they could do it very quickly—the Pathfinder Core Rulebook was published in August of 2009, barely a year and a half after D&D 4e. In contrast, Wizards of the Coast had spent 3 years on the core rules for each of 3e and 4e. (It’s not exactly clear when Paizo decided to start working on Pathfinder—but it was unlikely to have been prior to 4e’s announcement in 2007.)
Pathfinder allowed Paizo to continue to publish their adventure modules, which were their real focus and interest. Extensions to the Pathfinder system (classes, feats, and so on) were enabled through low-paid freelancers with minimum editorial oversight, and allowed Paizo to keep Pathfinder “alive” through a blistering release schedule, again at relatively low cost. And so they could focus on selling adventures.
An aside: the “old-school renaissance”
Paizo wasn’t the only company to notice the fractured D&D fanbase. Numerous other games, labeled “Old-School Rennaisance,” or OSR, came out to try to capture those players who ditched not only 4e, but 3.5e as well. So in addition to Pathfinder being a spin-off of 3.5e, there are other games that are spin-offs of or inspired by older editions, from before Wizards of the Coast bought TSR.
The death of D&D 4e
The story of Pathfinder suggests that D&D 4e was a complete disaster; that’s certainly how many Pathfinder fans view it, and probably also Paizo themselves. However, it’s not really accurate: D&D 4e did well enough, and again did particularly well with new players, relative to Pathfinder mostly focusing on old players who didn’t like 4e.
As a game designer, I will also say that D&D 4e is easily one of the most tightly-designed-and-executed RPGs in existence. Other editions of D&D aren’t even playing in the same league. That’s not everything—4e was excellent at being itself, but that only matters if you like what it was trying to be—but a lot of the critique of 4e out there is based on rather superficial reads of the first book, rather than actual-play experience leveraging everything the system eventually offered.
But there were also a number of very real problems. Some of it was poor planning, some of it was pressure from Hasbro (which owns Wizards of the Coast) to cut costs on D&D, at least part of it was a murder-suicide (!) by one of the lead developers of a 4e virtual tabletop, killing not only himself and his wife, but also that project and a lot of Wizards’ plans for 4e.
In the end, 4e ended up losing support from Wizards of the Coast. They tried to semi-reboot it in 2010 with the “Essentials” line that was supposed to appeal to players turned off by the rather-complicated process of building a 4e character, but mostly it failed to appeal much to actual 4e players (who mostly did like the character-building process), and failed to get much attention from other would-be players (who were mostly playing—and mostly happy with—Pathfinder). And between 2012, when Wizards announced they were going to make a new edition, and 2014, when the fifth edition was actually released, there was basically no new D&D content at all (unless you count the playtest packets for “D&D Next” as the beta system that would eventually become 5e was called). So even if you liked 4e, sooner or later the fact that there was new Pathfinder content and no new D&D content meant a lot more people switched to Pathfinder.
D&D 5e: A return to form
Released in 2014, D&D 5e was an attempt to recapture the player base that had been lost to Pathfinder and the OSR. It undid a whole lot of changes made by D&D 4e. It embraced an “old-school” playstyle to a large extent. It also put a huge emphasis on being simple, easy to learn and play, and welcoming to new players, which is not something any of the other games mentioned here can say.
And it has been extremely successful.
No numbers are known here, but Wizards has claimed that recent years have been some of D&D’s best—and that goes all the way back to the original editions in the ’70s and ’80s that became an international phenomenon.
Pathfinder 2e: A surprising departure
In 2019, Paizo released their second edition of Pathfinder. It is surprisingly 4e-like in a number of ways, which is somewhat ironic considering that Pathfinder was written as a response to 4e in the first place. (It also has a number of 5e-like features, and of course a whole lot of it is unique.) Perhaps most notably, it’s a large departure from PF 1e, greatly changing the game in a large number of ways.
This has been controversial. The system is still quite new, and the verdict is still out on it, but there is a risk here for Paizo that they will follow in 4e’s footsteps—clearly not their goal. Time will have to tell on that, though.