How to be a Likable character

[Kromm]

Like it or not, there's also a "meta" issue lurking here: As the GM, if you ever roleplay an NPC as useful, friendly, and trustworthy for a long time (say, for many real-world months or tens of encounters), and then suddenly have said NPC betray the PCs for any reason other than coercion (e.g., the NPC's family is threatened) or mind control, rest assured that any future group that includes any of the players who witness this is unlikely to trust or like any of your NPCs ever again. This is especially true if the PCs don't get a chance to use Empathy, Mind Reading, Body Language, Detect Lies, Psychology, or similar capabilities to spot the heel turn coming.

Thus, a good rule to follow is this: "If an NPC is genuinely likeable and friendly – to the point where even untrusting PCs can find nothing to worry about by reading minds and analyzing behavior – then make sure that NPC stays likeable and friendly forever after. Betrayal should happen only as a result of the NPC becoming a victim in a way that lets the PCs rescue him and then get revenge on those who put the proverbial gun to his head." A corollary is that if the PCs rescue an NPC from villains who coerce and control people, they'll often tend to like that NPC and "adopt" him.

Yes, realistic people change loyalties, have bad days, and can be alienated and seek to settle scores. These reasons aren't good enough for NPCs.

All of the above goes only for individual NPCs who are inferior to or at best equal to the PCs in competence, and who are acting on their own accord, not as the face of an organization. Players generally know and accept – still on a "meta" level – that high-powered CEOs, generals, high priests, kings, etc. have ambitions and engage in machinations. Being betrayed by someone like that is "just politics" and usually not grounds to never trust your NPCs again.