Gunner found Zero on one of his jobs. He was part of a team to take down a mercenary groups ambitious ideas of genetic manipulative within a deep space laboratory. Zero was one of there experiments. Only a young boy then his rapid growth and ability's have made him a natural hand at combative work, something Gunner has taught him along the way.
Zeros got a specially adapted stealth suit whilst gunners always been the tank..... they work great together.
Interesting place to keep a sword, but how do you draw it? I mean, imagine having a sheath taped to your chest and try pulling out the blade... Not that easy or smooth, is it? Not to mention that it leaves your sides wide open!