u/AK42104

The blue-chest design of the Xi Gundam first appeared in the game SD Gundam G Generation F (2000), as seen in the first image. It wasn't actually Katoki’s; it was an updated redesign by Yasuhiro Moriki himself for the game. This version maintained the stout proportions, shorter legs, and oversized hands familiar from the latest film.

This is similar to how the Hi-ν Gundam was redesigned by Yutaka Izubuchi in 2007 with a leaner, blue look—despite Izubuchi also being the one who designed the original, bulky purple version in 1988.

While Hajime Katoki’s specific Gundam Fix Figuration (GFF) redesign debuted in 2004 (fourth image), the switch from the original white chest to blue had already been introduced four years earlier by Moriki. Katoki did, however, streamline the silhouette, adding his signature "supermodel" proportions: long legs, sharper edges, and a smaller head and hands.

Twelve years later, Katoki updated his Xi design again for the Robot Damashii line. He added a new weapon—a Micro Missile Pod—which probably acts as a tail stabilizer and adds extra firepower (last image). He also simplified the aesthetic, using fewer details and decals.

The 2021 film captured Moriki’s original vision through mechanical designer Seiichi Nakatani. While not a direct 1:1 copy, the proportions remain faithful. Katoki then "cleaned up" this movie design to make it easier to convert into model kits. He did something similar for the Crossbone Gundam units; while Yuuichi Hasegawa originally designed them, Katoki refined the "wonky" illustrations of his into the more mechanical, presentable forms seen in the HG and MG kits.

With that being said, when the third film rolls around >!and for those who have already watched the second film, given that the Xi was damaged during its sortie with the Alyzeus/MP Nu in the second film, it will likely be repaired for the final battle in Adelaide against the repaired Penelope. !<It’s possible the suit will sport Moriki’s updated G Generation F design, finally bringing the blue chest to the big screen and also another model kit for Bandai to sell.

To summarize:

• Original Novel Design (1989): Designed by Yasuhiro Moriki; featured a mostly white body (third image).
• Game Redesign (2000): Moriki refined the Xi for SD Gundam G Generation F to look like a "typical Gundam," introducing the blue chest and Gundam Face.
• Katoki GFF Redesign (2004): Hajime Katoki later refined this "blue-chest" game version for the GFF #0025 figure. His version added the "Ver. Ka" sharp, squarish aesthetic and specific decals that fans now heavily associate with the blue-chested Xi Gundam.
• Movie Version (2021): The recent Hathaway film reverted to a white chest and a more "monstrous" look to honor the original 1989 novel illustrations. This was designed by Seiichi Nakatani and cleaned up by Katoki (second image).

The blue chest originated in the 2000 game SD Gundam G Generation F, but it was standardized and popularized as a "heroic" look by Katoki's 2004 under GFF line.