if you add a %default, %damager, %killer, %victim, or %uuid, they will be replaced with the corresponding value, so they will be: the default player's name, the damager's name, the killer's name, the victim's name, or the selection's uuid. I recommend you using %uuid for saved stuff just incase the player changes their name; now to use these:
get a variable and call it %killer killstreak, this will be automatically renamed to (for example) Pancake killstreak, or ARMcPro killstreak depending on the killer, so you can then use %default killstreak (for loops, for example) or %killer killstreak (to send a message in the Kill Event, for example) as...