The Internet has revealed the three longest Japanese Wikipedia articles by character count, and surprisingly enough it is a video game article. Number one on that list is a page dedicated to the characters in the Trails series, clocking in at about 440,000 characters, according to Yahoo Japan.
Known in Japan as the Kiseki series, the Trails series are series of games created by Nihon Falcom that are a spinoff of their Legend of Heroes series, which began seeing releases in Japan in 2004. The series has several distinct major story arcs, such as Trails in the Sky and Trails of Cold Steel, that all focus on different places on the continent of Zemuria. In general, the games all take place within the same general time frame, which means plenty of cameos and recurring characters. America got its first taste of the series in 2011 when XSeed Games started releasing localized versions of them until NIS America took over localization in 2019.
In Japan, the series right now has 11 mainline titles, spanning across four arcs, each of which have a mixture of new and reocurring characters. Being JRPGs, many of them have a number of appearances and activities that they have done over the past decade and a half that the series has been seeing releases over. Trails in the Sky and Trails of Cold Steel have been translated, but the two titles between them, called the "Crossbell arc," which takes place in the titular location, have yet to recieve official translations. However, fans have created fan translations of the two games (Zero no Kiseki and Ao no Kiseki.) The two most recent releases, have yet to see English translations, and are part of the fourth arc.
As for the Japanese Wikipeda page, Yahoo! Japan reports that the characters page for the Trails series has 440,000 typed characters. In addition, it has 326 annotations, 405 sources, and an additional 265 secondary sources. Taking second and third in that article are the characters page for the manga GeGeGe no Kitaro at 380,000 characters and "TV in 2020 (Japan) with 350,000 characters. A few of the runner up articles include the episode list for Gogol 13, Yukio Mishima, and the history of the separation of church and state in Europe.
Tales of Cold Steel IV, the latest of the localized games and the end of the Tales of Cold Steel arc, is currently available on PlayStation 4. A worldwide release for the Nintendo Switch and PC are expected to arrive this year.