Witness Rachel joining the group
Kill a mutant with your fists
Use your first Smart Bomb
Kill a zombie
Complete all character quests or see someone else do it
Complete 'Return to the Military Base' or see someone else do it
Complete 'Dev Card Hunt!' or see someone else do it
Kill 500 zombies in one level
Complete a Base quest without taking any damage
Fully upgrade a weapon with any status effect
Witness Brian joining the group
Witness Luxo joining the group