She sounds like she's been introduced. Although, I cannot help but notice that your bullets for her don't list another PC. I have 4 rough stages of character development.
StageOne: Imagine them on the scene. (Ideas)
This has to happen for anything to ever involve an NPC. Sometimes you can do this over time, name them first and drop them in somewhere later, but both are part of stage one. NPC's at stage one are Ideas.
Name them.) There are three guys sitting on stools in the bar: Thumper, Mix, & Freddy.
Place them.)
Thumper is a big balding laborer drowning in cheap booze.
Mix is a skinny weasel little fuck, counting through his bag of loot with a shotgun on the bartop.
Freddy is a manly man, with a glorious pitch black lumberjack beard. He is on the stool in front of his own bar, bored out of his mind.
StageTwo: Introduce them. (Pawns)
This happens when 1 of the PCs has a connection to an NPC. This can happen through the player's input, or by player choice. The MC can also prompt a relationship, but it's only really introduced when the PC picks up and adds to it. You'll know when a PC has been introduced for real. And you are right, like with this Girl-- they are someone you want to use again, but. You first and primary goal should be making that NPC move to stage 3. Pawns are just pieces to move around the narrative, they're color, distractions, hooks, and reactions.
So lets say when the [PC] Gunlugger walks into the bar, we do something like:
Hey [PC], you know all three of these guys. One of them has been your friend forever, you've destroyed things another one cares about, and the one is absolutely terrified of you from that time you shot up his gang. But I forget, which is which?
Freddy turns around and groans, "[PC] I told you never to come back here. I'm not cleaning up any more of your fucking corpses!". Ignoring him... Mix sees the Gunlugger and gives him a big grin, "Heyo Buddy! Get over here, I've got some really cool stuff to show you." The drunkard turns, looks towards the [PC] in horror and screaming... he bolts out the back of the bar immediately, stiffing Freddy. [PC] flicks off Freddy as he walks over the Mix to see the cool loot. (they start talking) Freddy cocks the bar's shotgun and aims it at the [PC] "I said get the fuck out.". Mix chuckles, "Alright alright buddy, let's go someplace else" But... What do the [PC] do?
Basically, We have some color now, and the player picked who was who, so now they've had a say in these people's history with them. Maybe this is done with opened ended or leading questions, maybe just picked from the list. Often times it'll happen purely by player action when they take the initiative to interact with someone new and then do something that has consequences for that person.
StageThree: The Triangle. (Characters)
At this stage, you or one of your players has imagined the NPC, and you've introduced them on the scene to at least 2 PCs. Now I don't mean at the same time, If both PC's have the same relationship with the NPC then this doesn't count. The NPC needs to be shown to both PC's at a different angle.
Let's say Mix is a Character. Because the Savvyhead [PC] has also had him on the scene. We learned when we were talking to the [PC], that Mix is some sort of free-wheeling hard drug dealer, not a nice guy. So we start a scene with the [PC] hearing a scream from just outside his workshop, he grabs a claw hammer off a tabletop and rushes out to find Mix beating down some woman just around the corner. Mix is snarling, "You Traitorous little bitch!" Kicking her in the ribs, and again, and again. She's gasping for breath, what does [PC] do?
Assuming the Savvyhead played the hero and did something to stop what he was seeing. Maybe he threatened Mix with the clawhammer, and Mix thought better of it. Maybe he threatened Mix with a claw hammer, and Mix kicked the shit out of him too. So long as we have some tension, it's all good.
Now, Mix is a character. What happens to him might piss off his best friend the Gunlugger. However, the Savvyhead might not tolerant this fucker anywhere near him. Sweet! Characterization achieved. Now when this guy moves around, more then one person is watching. This is where you want to get as many NPC's as you can.
StageFour: Develope them. (Story)
You will find that things will happen to your characters over time, and their relationships they've established with the PC will begin to change. When they changed enough that the person you (and everyone else) thought they were at the start, isn't really who they are now. You've hit this point.
(The following example is from one of my games, this was not the focus of the game. It took place now and then over the course of everything else)
I once imagined an NPC for a [PC] Hardholder, Dallia, his second in command, as being hopelessly in love with him, and that translated into going above and beyond in her job. Dallia, well, she had met and enforced the "law" on all the other PCs in her own way, with varying degrees of success. She was even recommending to the Hardholder that one of them be "dealt with". She was a character now. Everyone knew her and had an opinion.
However, the PC Hardholder didn't notice her affections, or didn't take the time to care, or just decided not to act on it, and got involved in countless other characters. One of those flings was particularly bad, flaunting him in front of Dallia on purpose. So Dallia uncharacteristically retired to get smashed at the bar, where another PC lived (the one she hated the most and had recommended the [PC]Hardholder get rid of, ironically). She sort of asked for advice, without saying exactly what the problem was, and the other PC told her to take what she wanted and not let anyone stand in her way. (He was that type of PC) He didn't know what she would do next.
So the drunk and pissed off version of Dallia decides he's right. She pushed her way out to the bar, slamming into one of the other [PCs] as they entered. That one was the squirrelly curious type, so he snuck after her spying. This set up everything, honestly. With a witness, this kept snowballing. Anyway, Dallia went and found MsFling, and butchered her in the bedroom, taking for herself a valuable neckless the [PC]Hardholder gave to her as a gift. The Witness freaked out, but Dallia caught him too, and dragged him into her lies. They framed someone else (by accident mostly, they just pushed blame and it fell squarely on someone else), someone another PC cared about (was actually in bed with) and the Hardholder went to war. The Other PC and the Hardholder spun out of control and became enemies.
Meanwhile Dallia was struggling to do her job, her performance in the pits. So the [PC]Hardholder replaced her with someone else, and told her to take a break (which is not what she needed). Then the [PC]Hardholder ended up in another relationship (marriage), this one a political arrangement between powerful Holdings. Dallia was spinning out, and went to accuse the PC[Gunlugger] that talked her into this. Instead of hurting him, he gave her someplace to be.
Over the next few sessions, she became more and more. The Brainer at war with the Hardholder got into her head and saw things about the past. The PC[Gunlugger] gave her a home, showed her what love was, and prepared her to stand on her own feet. The Witness, wary as ever, only started to come around, but he had a performing troop of actors, and showed her a place where Death wasn't the goal.
By the End of the game, she was a fucking badass NPC. She had loved, murdered, been broken, and mended. The thing with the gunlugger wasn't to be kept, it was just helping hand. During credits she did what she always wanted to do, she went to go find out what was beyond the horizon.
It was a very powerful little story. But that's when you know you've got a character in stage4.
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This is somewhat off-topic, I know.
The point was to showcase how I approach development. When you've got bunches of triangles all strung together, you can push on one and they slam into the others. The entire network bends and pulls like a web someone stuck their finger into. It becomes easy to get complicated realistic reactions without having to do much work. That's how I run a game anyway.
What I didn't show here, is all of the other triangles that made this possible. So what I recommend for you to do with your love letter, is not to try to answer your stakes on that Girl within love letters. But instead, since this Girl sits at the center of a web already attached to a network of NPCs... and they're all ready to do something depending on what happens. I would reinforce those stakes by drawing triangles between the various sides to your players. Get them involved. Have Ambergrease come around with a big fucking gang to see how she is doing, make sure the Hocus knows that this guy might shoot them all if he feels betrayed, or if he can't trust the Hocus. Now the Hocus has to decide whether or not to admit the kidnapping to him, or if he's got to cover it up. Tensions! Have the one that talked the 5 into going to fetch her be involved, or maybe have to go after the 5 and help them, or pull them a different way.
You might want to draw some triangles to the Dread Teeth too, maybe let one of them get found out somewhere the others arent, some teen or something. Someone that isn't immediately ruthless, probably more vulnerable. Let a PC interact with them, show the moral side of them. Make them human by making getting them to sort of like each other. Maybe talk about people in the gang, speak highly of them, the actually good things they've done for the kid. Enforce what threat they are, but make sure that another side is seen before the funs go off and people start dying.
Now you've got fertile ground for anything to happen.
Until you've got triangles with someone, developing them is just an arbitruary decision. It doesnt really count. Do it for sure, if you need to, but it's best when someone is there to witness or better yet, trigger the WHY. I guess.... The real reason I wouldnt use my example as a love letter, is that I would want to set up all the other scenes before resolving it. Skipping to that makes it impossible to use as a source of narrative tension.