Seeing as all the first pieces are now out, here's how all the YNs in the Fly Me To The Moon series got their nicknames.
Angel
It started when you were younger, specifically in middle school before Bradley hit a growth spurt and bulked out from playing baseball. You're 12 years old and a little taller than Bradley, you've got a mean right hook and a bad temper. Some older kids from the next grade up were bullying Bradley about he was too small and scrawny to be a pilot, how he would never live up to his dads legacy, and you just see bright red and swing for them without thinking. You break 2 of their noses and the other runs away before you can land a punch. You end up in the principals office, your mom at your side and though she's acting strict in front of the principal as she's dishing out hour week long suspension, she's sending you a wink so you know she's not actually mad at you. Its later that day when you're home, Carole and your mother drinking wine in the kitchen while you and Bradley stargazing out the back, that Bradley says your like his guardian angel always looking out for him. He makes little comments about you being his guardian angel, or simply just being an angel ("God (YN), you're an angel" is a frequent saying), throughout your middle school years but it doesn’t really evolve into a nickname until freshman year in high school and he starts to call you angel unconsciously saying things like "thank you angel" when you hand him a water bottle after baseball practice, or letting out a begging "please angel?" as he begs for help studying for a test, even starting to greet you with a "hey there angel". You stop blushing at it after 2 months, your heart stops racing after 3 and after 6 months you barely even register that he's calling you it anymore. By the time senior year rolls around, people are convinced Bradley doesn't even know your real name with how often he calls you angel. After you and Bradley start dating, the dagger squad also start calling you Angel stating it is now your honorary call sign.
Peaches
You grew up with Jake Seresin, your family orchard and vineyard only a few miles down from his family cattle ranch. When you weren’t helping around the Orchard and Jake wasn’t helping around the ranch, you would often spend time on each others land, either running around the wild flower field on Jakes ranch or star gazing on top of the barn, or climbing the apple, plum and peach trees in the orchards or going horse riding through the vineyard and out into the open land your family owned. You were best friends throughout your entire childhood and moving into high school you were the typical pairing of the star football player and cheerleader that were rarely seen apart, to the point that everyone thought you were dating and you constantly had other cheerleaders asking you if Jake was available and other footballers (as well as guys from the other school sports teams) asking if you were available. It was your sophomore year when Jake started to realise he may have thought of you as little more than a friend he also noticed you always smelled like peaches, most likely from your body wash or shampoo. He also realised that your lips tasted like peaches after a game of spin the bottle at one of your friends birthday where he finally got to kiss you. After that he started to call you Peach or Peaches and when you asked about it he simply shrugged with that cocky grin of his on his face saying “because you smell and taste like peaches”, causing a huge blush on your face, which only caused him to continue using the nickname throughout the rest of high school and into your adult life. When he finally introduces you to the dagger squad, they only know you as peaches as that what Jake used when talking about you so they all call you peach as well and you don’t mind it.
Siren
You and Bob had known each other since the naval academy, meeting on your first day. He was ever the wall flower, probably more so back then, and it was the classic trope of the introvert (Bob) being adopted by the extrovert (You). It was good for Bob, you instilled a sense of confidence in him that he didn't have before and he came out of his shell little by little (mostly just around you). You were attached at the hip and followed each other to Top Gun, where you were considered to be top of your class. It was on a night out with your class, at the Hard Deck (before Penny took over), that you got your call sign. The Hard Deck was doing a karaoke night and the other members of your class had made a bet that no one had the balls to get up and sing, all putting in $20 each for the brave soul to go up and sing. Never being one to back down from a challenge, and being confident in your singing talent, you wink at Bob before going up to the karoke machine and belting out a killer version of 'Hit Me With Your Best Shot' that leaves everyone in your class shocked, and Bob absolutely mesmerised as he watched you perform. Its after your performance that someone makes a comment that you had Bob in a trance (causing a blush to flare up on his face), like a siren luring a sailor to their death, and thus your callsign was born.