If there is no face-up "Kozaky" on the field, destroy this card. If this face-up card on the field is destroyed, inflict damage to the current controller's Life Points equal to the original ATK of this card
Another one of Kozaky's inventions, this one you actualy need Kozaky to use. 2500 attack beats Rare Metal Dragon's 2400 for a 4 star monster, and it's slightly easier to summon, but with 400/400, Kozaky (and Giant Kozaky) probably won't last long.
But if Kozaky is destroyed, Giant Kozaky goes with it dealing 2500 life points, which isn't good at all... But, there are ways around this!
Heart of Clear Water: Makes Kozaky indestructible enough for Giant Kozaky to last longer
Creature Swap: Switch control of Giant Kozaky to your opponent, and use the monster you got and Kozaky for a tribute summoning (or just destroy Kozaky somehow) and you deal 2500 damage to your opponent, and leave him one monster short!
Skill Drain: The problem here is that if Skill Drain goes, you take damage up to 7500 if you were using three
As a minor added note, it doesn't say it can't be summoned without Kozaky on the field, but why you would want to give up your summon to take 2500 damage is beyond me... unless your opponent plays it with Conscription, then they take 2500
In the end, it's a powerful 4 star monster that relies on a weak 1 star monster... but it could be very good if it had it's own deck built around it