Metroid - Zero suit bounty hunter

That soulless eye

If her character was ever fleshed out in a realistic way (not Other M), I can imagine she'd be a very broken individual.

Her parents were brutally killed by a space dragon in front of her when she was old enough to remember it. She was raised by weird bird aliens who have now all but vanished. Most of her missions involve her going into the depths of space hell all alone and fighting giant monstrosities. It's a lot of shit for one person to go through.

Soldiers can usually only fight for so long until they experience "battle fatigue", meaning they're just too emotionally drained to fight effectively. Some people don't really experience it as much, if at all, and it's usually because they're psychopaths.

TL;DR: Because she's been through so much shit and can still do her job, she's either a psychopath, or she's learn how to think and feel like one.