The NPC will help the players too much, helping or giving to the point of self-sacrifice. He secretly hopes to be recognized - even pampered - for this effort, but will never say so or let on in any way. Unless this happens, the NPC will feel resentful, and eventually come to feel utterly betrayed and neglected by the PCs.
2 Authority Issues
Though he doesn't realize this, the NPC is deeply mistrustful of authority figures. She will feel that attempt to suggest a course of action, however gentle, is an attempt to impinge on her freedom.
3 Passive Resistance
The NPC is resentful of any attempt of the players to push their own agenda, but won't speak up for himself. He will agree to anything, but whatever he's said, he'll privately do it his own way - or not do it at all. No issue is too small, or too important (e.g. ensuring the rations are packed) for him to take a stand.
4 Fully Entitled
It never occurs to the NPC that other people might have wishes other than her own. She blithely assumes everyone sees things her way, even when meddling in others' affairs. The NPC is quite comfortable with the resulting conflict, and sees others as oversensitive.
The NPC is secretly furious that nobody asks his opinion. He will never offer it unsolicited. He will hold his tongue, even to the point that the group fails. Only if pressed will he reveal that he had foreseen this disaster and that somebody should have asked him.
The NPC is desperately insecure. He perceives any contrary opinion as an attack on him personally, a deliberate move to either undermine him, make him look stupid, damage his reputation, or pave the way to pushing him out the group.
The moment he is inconvenienced, or anything whatsoever goes wrong, the NPC will immediately blame anyone plausible - preferably someone nearby. Their fault could have been something done wrong, something not done, something they didn't anticipate needing to do.
This NPC must be in control, and views this as the only possible route to success and failure is not an option. Independent action is at best an annoyance, one more thing for her to keep track of, and at worst, a first step to disaster. She will resist this with great determination. No detail is too small for her to have an opinion; plans too complicated for her to track in her mind are a non-starter.
No matter how urgent the information, this NPC will always try to communicate through an intermediary. "Can you talk to him? It'll sound better coming from you." If he doesn't like someone in the group or their behavior, he will speak to the others one on one, trying to turning the others against his target. If confronted directly, he'll deny that he has any gripe.
The NPC holds a grudge and keeps score. Any annoyance, perceived slight, failure to recognize her contribution, or infringement of her rights will be endured silently - but every last one is stored up in a mental logbook. Then, at the least convenient moment, she snaps. Maybe it's one slight too many; maybe a player needs a favor.