Chaser are one of the latest creations of KronoteK. Used for KOing enemies with blue auras. Try to only use them if you really need to-you only get a limited amount each level, and when they explode, they're gone. You get around 10 new Chaser each mission. They are one of the two ways of subduing enemies with blue auras. (The second method being, shooting them in the legs to keep them alive.)
Chaser are used by throwing at enemies with blue auras to knock them unconscious, as you are not meant to kill enemies surrounded by a blue aura.