Code names

I am currently needing suggestions for code names. I want something unique and that fits with his/hers personality or specialty.

I have four main characters that need code names. Then I have four others whom I have not yet decided who they will be. so a total of eight. The other four can be anything really, I'll just use what I see fit for those characters.

But the main characters are

Andria: Female/ Sniper

Mike: Male/ Squad Leader

Sam: Male/ Intelligence 

Korra: Female/ negotiator

Any suggestions will help. I'm also thinking about creating a twitter, any thoughts?