Some of the codenames I've come up with have been somewhat uncool, but the point was to use something that's connected to the character, something they'd naturally have. This potentially makes it easier for your readers to remember whose who. I'm saying potentially. Probably 90 % of the time they're going to get confused. Of course, since it's your story, your goal might very well be to go for something that sounds cool. Fair enough. Then you probably want to use names that have connotations to what you consider cool. I'd recommend perusing a thesaurus for more ideas.
@Lew @Wreybies What is the difference between a call sign and a nick? Doesn't a call sign serve to identify a broadcaster or something?
Yes, the above in bold is correct, but the term has been appropriated into more general use and you often hear it used in military and paramilitary films and shows meaning the same thing as just a nick. I've also heard handle ID (the 2005 film version of DOOM). There's a drive inherent to those who aren't inculturated to give names to these kinds of things to lend a feel of regimentation. In the military, no one would say "my nick is so&so" or "my handle ID is such&such.". That would sound so artificial. Like someone is trying just a little too hard to be GI-Joe. You would just say "they call me so&so".
