The "Kage" portion of his name is derived from the Japanese word "kage" 「影」, meaning "shadow," while the "maru" portion means "circle" or "perfection". Therefore, this character's name could mean "shadow perfection". Another interpretation is "shadow of perfection," perhaps referring to his failed attempt at becoming immortal, or "perfect". "Maru" is also a common end for male Japanese names.
He was first involved in the series as an unseen figure talking to Professor Banner through his computer screen, telling him that it was an appropriate time to test Jaden in the Gravekeepers world. Banner argued Jaden might not be ready, but Kagemaru said it mattered not, and he simply would have no use if he failed.
He is next seen with the Shadow Riders, asking which one would make the first move, with Nightshroud revealing he would act. After Dr. Crowler loses to Camula, he appears on a screen in Chancellor Sheppard's office, taunting him that two keys have already been lost, and wondering if anymore students would fall to the vampire.
In Titan's flashback in how he escaped the World of Darkness, Kagemaru was shown reaching out to him giving him his item, and promising him freedom if he would serve him and defeat the key holders.
After the seventh Rider, Amnael falls, he finally makes his existence known to Jaden and the others, being air-lifted in via helicopter, in his stasis tube. Many students challenge him to a fight, however he only accepts a Duel with one student - Jaden. During the Duel, He Summons all three Sacred Beasts, and they gradually absorb duel spirits which eventually restores him to a muscular much younger form. Jaden struggles against the Sacred Beasts, but ultimately manages to come out on top. After being defeated, he returned to his aged state, and promised to renounce his former ways and asked for forgiveness, which Jaden was happy to give. Kagemaru was then carried to the hospital via helicopter after Jaden accidentally crushed his spine with a bear hug.
Kagemaru returns to Duel Academy with Sartorius to tell Jaden that they have both had some sort of premonition, feeling that Duel Academy might be in grave danger, theorizing that the Sacred Beasts, Light of Destruction, and Yubel incidents might be related. Both Kagemaru and Sartorius are later present at Jaden's graduation ceremony.
In Yu-Gi-Oh! GX Tag Force he serves as the final boss of the game, and the player is forced to duel him twice in a row, once in his older, machine-bound form and again in his youthful form. After completing the game, he can also be found during part 1 on the cliff after 10:00 PM. Upon acquiring his Deck Recipes, he also gifts the player with a "Phantasmal Martyrs" card.