There is a fine line in roleplaying games where a nonplayer character (NPC) flips from being a reasonable part of the fictional game world and starts generating antipathy from the players. There’s characters you love to hate and there’s characters you just hate. Hopefully we can stay away from the latter.
“You’re not worth killing” might be harsh, but think about it from someone on the receiving end: you’re not getting killed today! Surviving because you don’t present yourself as a threat is a tale as old as time. This approach is great for informants, minions, viziers, and other NPCs who haven’t been presented as someone with the physical prowess or motivation to personally defeat the player characters.
In a combat situation, this can be similar to not being a threat, but an NPC with an egregious flaw that evokes sympathy or humor is a lot less likely to get murderhoboed. Dewey Crowe, from Justified, survives so many seasons despite being a pretty terrible person because he’s just that bad at being a criminal. Also, it’s a scripted show so it’s not really an applicable example but whatever.
From purposefully incompetent, we move onto personality traits and situations that make an NPC fun to have in the game. As the GM, you’re trying to differentiate NPC from NPC, trying out manners of speaking or describing interesting fashion choices. Sometimes this will resonate with the players, be it a rude waiter with a penchant for getting orders exactly wrong or a genteel vampire with a penchant for feathered hats.
The enemy henchwoman who puts up a challenge and who isn’t clearly built on fiat, and who respects a PC in combat might get a pass. This one’s tricky, and is a bit reliant on emergent outcomes during play. That is to say, you can’t force a sense of fair play but you can certainly sabotage it before it’s had a chance to come to fruition.
I’ve found that recurring NPCs are less work for you and are more likely to survive in order to show up again. As the party learns more tidbits about the NPCs they interact with, these imaginary people bring more life to your shared world and more feels to your campaign. It’s a heavy stone, but once it gets rolling it’s easier to keep it moving.