The shadow creatures (Japanese: 影 Kage, lit. "shadow") appear in both Ico as enemies to be defeated and Shadow of the Colossus as spiritual entities representing something more. Their inherent connection between the two games is not obvious, but is still present.
In Shadow of the Colossus, they represent lesser foes, shadowed by the idols and their related colossi. In Ico, they are vicious enemies bent on taking back Yorda for their Queen. While their true nature is hidden, they play an important role in understanding both games.
In Ico, the creatures arise out of portals in the floor and charge Ico in an attempt to capture Yorda. All horned, they come in several varieties: some fly, some are bipeds, and some skitter around as bugs. While only the humanoid ones can attack Ico and snatch Yorda, the smaller, insect-esque creatures become humanoid if the others have fallen, making them formidable foes in the quest of Ico.
The shadow creatures are fixed on taking Yorda into the portal from which they came, and they only attack Ico as a means to an end. They can be cleared entirely from an area in some instances, though in other times they must be fled from. All shadow creatures can be vanquished by Yorda's unlocking of an idol door. If Yorda is left unattended in another area for too long, a single shadow creature appears to abduct her.
The origin of the shadow creatures is not disclosed in Ico, but much speculation arose over their appearance in Shadow of the Colossus (see below). Clearly they are servants of the Queen who wish to bring Yorda back to be used in the Queen's ultimate purpose: self-preservation. Their shape suggests some kind of spiritual oddity: not quite human enough, not quite beast enough.
In one of the final scenes of the game, shadow creatures in the forms of young boys similar to Ico appear. It is almost a certainty that these are the other horned boys who were captured and sacrificed to the Queen. This seems to indicate that the shadow creatures encountered throughout the game were the other offered boys all along. The behavior of these horned shadows is aberrant at best; some of them attack Ico, only to flee at the sight of conflict, while others fly in circles with each other, paying little attention to Ico. There are 39 total; Ico would have been the 40th had he perished in his tomb.
These scampering creatures will congregate around Yorda, but will not attempt to capture her. However, if Ico defeats a larger spirit that is nearby, they may morph into a replacement and launch an attack.
Sentry Spirits attack with their flightless wings. They will often try to separate Ico and Yorda before picking her up and walking into a portal. Sentries are difficult to defeat as they can create protective shields around themselves.
These huge, hulking spirits have the power to knock Ico across a room. They sometimes attack in pairs with such violence that Ico will find it impossible to counterattack. One of their main tactics is to back him into a corner so that he cannot protect Yorda.
The Sentinels are a pair of fierce and deadly apparitions. If they manage to capture Yorda, they will use their powerful wings to fly her out of Ico's reach.
They appear only at the end of the game in the pod room of the beginning of the game. Their behavior is inconsistent, though for the most part they ignore Ico and only surround and stare at Yorda. Some assault Ico, others flee in the face of dispute, while others fly in circles together. It is likely that all the shadow creatures that are encountered in the castle are these spirits in alternate forms.
Shadow of the Colossus
The audience's first glimpse of the shadow creatures in the "spiritual prequel" to Ico is in the very opening scene as Wander arrives in the central Shrine of Worship. In his essay Antagonism and Continuity, Bruno Figueiredo describes the scene and its inherent connection to Ico like this:
"When Wander reaches the temple and places Mono over the altar, some shadowy figures appear before him. These are clearly a resemblance to ICO’s enemies or, as we know now, the dark essences of fellow horned boys, refined and under the possession of the Queen. As they appear 'rom the ground, Wander swiftly beckons Agro to move away as he raises his sword, reflecting a light which dissipates these creatures. At this moment, the skies are filled with thunder and a god, answering by the name of Dormin, awakes. He is immediately aroused by the sword Wander possesses and recognises that weapon for some unknown reason.
It shouldn’t be too unreasonable to affirm that these mystery shadows are indeed the first step of a pre-arranged trial. Our character has the sword, so we proceed, but let’s imagine, for a brief moment, that he didn’t: this might have been an obstacle harder to outdo. The sword is much like a key in this land, awakening Dormin, reflecting the light and defeating the Colossus. And this is where the first connection with ICO appears. These foes are very similar to those who were seen in ICO. This, I think, is not without a meaning. Ueda was trying to remind us of those foes from his previous game, so troublesome and hard to defeat, at times. Well, this game is a whole new story... Wander defeats these with no trouble. He is warning the intelligent player that a whole new challenge is to come, much harder and... larger, eventually."
With that being said, the role of the shadow creatures, in terms of conflict, is ultimately diminished in Shadow of the Colossus; their role is a much more subtle one. As stated above, hints are gathered to the origins of the shadow creatures in this game. Each time Wander defeats a colossus, he is transported back to the Shrine of Worship where he is revealed as unconscious, surrounded by a number of shadow creatures equal to the number of colossi defeated. From this came the idea that the shadow creatures represent the spirit of Dormin broken and stored in the temple idols, now free thanks to the defeat of their related colossi.
Additionally, in the closing scene, Wander is possessed by Dormin. His appearance is that of a huge, shadowy creature. After the closing sequence, Wander is reverted to the form of a baby with horns, and hence comes the theory that Wander and Mono produce the first generation of horned boys, the progeny of whom would become the protagonist Ico. Moreover the shadow creatures seen in Ico are presumably of the same breed as those who inhabit the Shrine of Worship in Shadow of the Colossus.
It should also be noted that the official website (UK Version) states that, referring to the shadow creatures, "Everything casts a shadow" and also that "when an entity exists beyond the mortal realm, a shadow is all men can see". This confirms the nature of the shadow creatures to be supernatural, as they are the shadows of spirits which exist in another dimension or realm.