- Acclaimed Flop: Reviews and players immediately fell in love with the game, but it was way too unique and strange to be a big seller. The fact that it got almost no promotion didn't help either. The game's infamously hideous North American cover art is also considered to be a major reason it flopped in the US.
- Children Voicing Children: Kazuhiro Shindo, the voice of Ico, was only 13 years old at the time of the game's release.
- Christmas Rushed: This is the reason why the NTSC version is stripped down compared to the PAL and Japanese versions of the game. Amongst the changes includes the co-op mode, the secret hidden ending, Yorda's speech translator, and several puzzles being either simpler or outright not present. There's also unfinished lightning and models throughout the game. This was all corrected in the re-release. The NTSC release being rushed is also the reason why it has such an infamously ugly looking cover; the actual box-art wasn't ready yet.
- Development Gag: The original enemies were horned soldiers. References to the characters still exist at the start when Ico is imprisoned. There are also a few statues of them scattered about the game.
- Moved to the Next Console: It started life as a late release Playstation title before being moved to the Playstation 2 to take advantage of the hardware.
- Relationship Voice Actor: Rieko Takahashi and Misa Watanabe were regulars on the One Piece franchise both before and after the game's development (Takahashi was Conis and Watanabe was Nefeltari Vivi).
- What Could Have Been:
- The game started development in 1996 and was originally intended to be a PS1 title. It was moved to the Playstation 2 because the game was too taxing for the console, especially Yorda's AI.
- The pitch video for the game depicts a hornless Ico in a t-shirt and shorts, while Yorda was older, had glowing tattoos over her body, wore different clothes, and had a horned helmet on. They were being chased by flying robots who shot lasers at them, however they escaped the castle and hid on a nearby deserted island.
- For a large portion of development, Ico didn't have horns. As far back as the pitch trailer, he was meant to take Yorda's horned helmet and wear it in order to stand out on screen. It wasn't later until Ico became a Horned Humanoid. Yorda went through multiple redesigns, especially when it comes to hair, such as her having Girlish Pigtails for a while in development.
- The game's plot was simplified from development. Ico was supposed to originally help get Yorda back home but this was switched to just helping her escape the castle.
- Initially, the enemies were horned soldiers. They were removed because it was deemed they didn't make sense in the game's narrative. Instead, shadowy enemies replaced them because they could ambush the player without much explanation, they don't have to explain what happened to enemy bodies upon defeat, and they were easy to differentiate from the protagonists. References to the soldiers exist when Ico is imprisoned and in statues around the game.
- During development there were attempts to make the game more "game-like" in fear of it being too unappealing to gamers. For a while it had a health bar.
- There was a side-quest where Ico finds a blanket for Yorda to sleep with, but it was removed because it felt forcing the player to find a blanket would ruin the flow of the game.
- There's a whole bunch of unused dialogue in the game's files, including ones from a period when it was intended for Yorda and Ico to understand each other. Translations of the unused dialogue and retranslations for some used dialogue can be read here.
- Prerelease footage shows that the enemies were supposed to break apart piece-by-piece when Ico attacked them.
Trivia / ICO