spartanjohn_117 - First LJ username I ever made, actually. Not very creative. He's the Spartan named John-117.
doesnt_afraid - Second account for Chief, comes from the "I think Halo is a pretty cool guy. Eh kills aleins and doesn't afraid of anything" meme.

withteethbared - Comes from Bella's old name her wolf pack called her before she was found to be a werewolf, Stands-With-Teeth-Bared-Never-Backs-Down being the rough translation into English. You can probably see why the others liked calling her "Bella" better. The SN suits her temperament and Ahroun-warrior-ness too.

helipack_jack - Jack has a helipack. It also rhymes, and has his name in it. Simple!

outta_this_army - What does Grif want more than anything else in the world? OUTTA THIS ARMY! :P This is also one of my earliest journal names.
the_team_sucks - Second Grif journal, from a Grif quote:
Church: "Thanks for the support, Grif. Way to be a team player."
Grif: "Hey, I gotta think about myself here!"
Church: "There's no "I" in team, Grif."
Grif: "Yeah, there's no "U" either. So I guess if I'm not on the team, and you're not on the team, nobody's on the goddamn team. The team sucks!"

obscura_prodigy - Vega's last name is Obscura, and he is a Zoid piloting prodigy. I like to connect journal names to the character's name or canon somehow if I can, and I've never seen another character with the surname "Obscura". So, it links to Zoids, Vega specifically, and says something about him. :)
runs_on_rage - The Berserk Fury is a very angry Zoid. It has a measurable gauge of "combat motivation", which can be understood as how high its will to fight is and basically how ragey it is at any given moment. Once it hits a certain threshold it will ignore commands (even from Vega) and basically go on a psychotic rampage against anything it sees as a challenger. It has a mind of its own, but most of that mind is devoted to battles, rage, and looking forward creepily to the next time it gets to fight.

f_u_im_a_human - Hisk's SN is a play on the "Fuck you I'm a dragon!" meme. She actually is a dragon pretending to be human, so the irreverence and the "I am totally human, guys, really believe me!" thing works for her. I like the humor with this one, she could've ended up with something really generic. I kind of which I could've figured out how to make a "both main characters from Dragon Heart" sort of joke work without it getting too "...wut?"-inducing.