Just a question but is the "Dark Space" dimension that Zato-1 and Eddie use to perform their shadow manipulation actually Eddie's true form? I ask because according to the Drama CD translation, Eddie grew to enormous size at one point while fighting against the God Beast series.
Zato’s “Dark Sentinel” form is an appearance that occurs when both he and Eddie merge and attack at the same time (not unlike his sword-form “Executer”).
Eddie is described initially as a Shadow Parasite… also sometimes called Shadow Organism. He is called a Parasite, but that does not entirely mean he feeds off of Zato, even if there is evidence to the contrary.
What it means essentially is that Eddie needs Zato’s body in order to live, though this may or may not be an extra toll on Zato’s lifespan, especially now, compared to in the past, when Eddie’s lifespan was limited by his programming.
Eddie’s appearance as the Dark Sentinel is more akin to a “combat form”… a weapon mode, basically.
Forbidden Beasts (Kinjuu) were derived from Gear Biotechnology… so it stands to reason that Eddie is classified as a living weapon and can achieve a “weapon mode” much the same as Gears do.
Despite this, Eddie admits to Zato (upon his birth) that he has “no true form” in an absolute sense, which essentially means that the forms he takes are the forms he is most comfortable with or chooses to be, somewhat like a relaxed muscle that was developed over time.
Eddie’s personality is derived from Zato’s… so in that respect, he takes a form that resembles the philosophical “darkness” in Zato’s mind and heart.