We all want NPCs to be memorable. And for the most part we sorta succeed. Sorta.
But sometimes we have more luck than we realise and an NPC gets, for lack of a better word, adopted by the players. They recruit the goblin, keeps the NPC cleric on the party, keep going back to a favorite fixer for jobs and goods, and in the end that NPC gets fleshed out and more useful to the party at large.
But it goes beyond that sometimes. At least for me. My groups have a habit of...ummm...creative NPC enhancement. Like some examples about what I mean?
A farmboy saved from monsters is not turned over to the local lord but rather recruited by the party. They train him in class levels as one of their NPC cohorts, equip and arm him, and generally use him as a party squire.
Magical weapon made of the soul of a little girl is taken care by the party and they go out of their way to keep her 'the sword' company. Sing her bed at night, etc. Think about enhancing her bonus towards an Holy Avenger so only nobel souls will wield her.
Exalted NPCs are recruited, forcefully awakened as essence users, taught geomancy, cross trained in magical martial arts and sorcery. Then finally armed with low level artifacts banged out in a day. (Gotta enhance that followers rating some how.)
Selective infection by choice strains of lycanthropy mixed magical spells to allow the user to keep their mind. Then slowly infecting outward to local population of now feral combat weres to defend against an invasion. WOW eat your heart out.
The trend continues, but in the end the players tend to reach a peek of power and then start experimenting. Everything from minor magical enhancements, full body alteration, unique training, etc. If a subsystem exists a game to grant someone powers, they'll use it. They'll track the NPC's sheets, development, and training...oh and they'll pay them handsomely to stay loyal. In many ways the players are turning the NPCs into extensions of their character.
Given time the players build small armies of NPCs and 'extention' characters. Games like Ars Magica call this "troupe" play and it's awesome. The catch is...at what point should the GM hand the NPC stats over to the players and let them run the NPCs themselves. In games like D&D it's easier. They have cohort and hireling rules. Games like WOD, GURPS, and Exalted get muddier. You have to be careful of doing this, especially if the NPC has a chance of moving beyond the party and maybe even turning on them.
Mind you, that's a big 'be careful' moment. Players HATE loosing pet NPCs. The Pokemon joke in the title is not used just for jest. Players grow attached, and jerking their pet NPCs around pisses them off really fast.
But don't limit this behavior. It's good. I means investment in the setting and the NPCs. Support the habit. Because in the end, it's fun to keep pets...even if they ARE technically people.