What platform did the original 2013 Kingdom Hearts χ launch on?

The original 2013 Kingdom Hearts χ launched on web browsers.

Kingdom Hearts χ, pronounced “chi” or commonly “key,” was a Japanese online role-playing game developed for PC web browsers. Square Enix announced it as the first browser-based game in the Kingdom Hearts series, and the formal service began in Japan in July 2013 through Yahoo! Japan’s game platform.

Players created a customizable avatar, explored Disney-inspired worlds, collected cards, and battled Heartless. The game was set in the distant past, before the events of the numbered Kingdom Hearts titles, and expanded the mythology surrounding the Keyblade wielders and the ancient Keyblade War.

The browser version is easy to confuse with later mobile releases. Kingdom Hearts Unchained χ launched for mobile devices in 2015 and was later renamed Kingdom Hearts Union χ. Those are related successors, not the platform on which the original Kingdom Hearts χ debuted. The browser service ended in September 2016.