Mine goes back years actually, before I even started playing Xbox.
When I was playing with Legos with my brother, we were using whatever minifigs we had, and I had just recently gotten the Imperial V-wing set. I decided to take that minifigure and dub him the generic "Pilot 26," and the number was completely arbitrary. I think I just wanted a nameless soldier to play with, idk.
Fast forward a year or two, I had started getting a couple Halo Megablocks sets, and in similar fashion I was playing with my brother. This time, I decided to name my Spartan Shawn. Just a name, not sure why. At the time I was also writing fanfiction (it was trash, I'm not gonna lie. Storytelling is not my forte unless it involves someone else's script), and I decided to elaborate on Shawn. Soon after, a writing assignment in school asked for us to make a character, and I further elaborated.
Spartan Shawn-107 was a Spartan II, average (for a Spartan) in height, bulk, and features. He wasn't that attractive, wasn't that muscular, and wasn't that tall, but he wasn't unattractive, skinny, or short either. I He had a bit more of an emotional side than other Spartans, and at some point in the war he befriended a Spartan III, Josh I think, who sacrificed himself so that Shawn could escape.
I guess one thing about the characters I do write is that I don't like them to be exceptional. I don't like overly dramatic backstories, with characters edgier than Shadow the Hedgehog. I don't like purely badass characters either. I don't like the characters like Chief, who are the literal best at anything they do. I like characters who don't stand out, but are just the middle of the pack, not overperforming but not failing at their job either. They're just like your average person, but like every other person in this world, once you get to know them they are so much more.
Now that the character himself is out of the way (lol), why not Shawn107? Well, I wanted it to be able to be asked, and really the callsign of Sierra-107 appealed much more to me. However, it was taken, so S1erra107 it is. I've been using the tag for the last +6 years, and really have had people assume I'm female over text chats, and it can be fun to have them go "OH I'm sorry" XD
TL;DR: Shawn-107 was an OC for a fanfiction I wrote way back in the day, and I took his military callsign, Sierra107, and turned it into a gamertag.