Shiori Hirose is a twenty-year-old woman, whom is one of the few remaining BOARD members after it was attacked by the released Undead. She helped the Riders by detecting the Undead on her Undead-searching computer. She seems to care for Kenzaki very much.
Kotaro Shirai is a twenty-three year-old aspiring writer, helping Kazuma and Shiori's mission by giving them hospitality. He loves to drink milk.
Haruka Kurihara is Kotaro's older sister and owner of Jacaranda Cafe.
Amane Kurihara is Haruka's nine year-old daughter and Kotaro's niece. She is a rude girl who has feelings for Hajime. She is the reason he discovers what it means to be human. In the Missing Ace ending, a heart-broken Amane became a sukeban at age thirteen.
Short for the "Board Of Archaeological Research Department", BOARD uncovered the seal Undead Cards and developed the first two Rider Systems.
Kei Karasuma is the President of BOARD. A 48-year-old professor who created the Rider systems 01 and 02 (Garren and Blade), as well as the Leangle System (03), while he was under control of Isaka. During his travels, he befriends Noboru Shima, whom he entrusted the Rouse Absorbors to despite knowing he was an Undead. He also aware of the side effect of Kenzaki's King Form as well as Tennoji's plans of the fake battle fight. 4 years after the series end he and Tachibana tried to cover 52 cards, when Albino Joker appeared and unsealed Undeads, who attacked and killed Karasuma.
Yoshito Hirose is Shiori's father and the one responsible for breaking the Undead's seal in a fit of madness. He died in the process. He created the Trial-series Undead Trial B in his image to protect Shiori, possessing all of its creator's memories. However, Trial B was reprogrammed by Tennoji to continue to monitor the fixed Undead battle and remove potential threats before he sacrifices to protect Shiori from Trial G.
Hiroshi Tennoji is the chairman of BOARD who intentionally set up the series of events in the fake Battle Fight and the creation of the Catergory Ace Keroberos, so that he could have his dream come true. This warped dream was of a new world order with himself as its ruler. In the end, he was murdered by the Griffa Undead after he was defeated by the riders with his dreams shattered.