Hiroshi Tennōji is a pivotal character in Kamen Rider Blade, initially presented as a benevolent figure but eventually revealed to be the primary antagonist and the mastermind behind the series' central conflicts. He is the founder and former chairman of the BOARD organization, which was established to study and combat the Undead.

Tennōji's background is deeply intertwined with the tragic events of the story. Three years prior to the main narrative, a researcher named Hirose Yoshito caused the Undead to be released, an incident that Tennōji claims was a genuine accident for which he resigned as chairman to take responsibility. However, this is a deception; the incident was part of his intricate plan. He used his immense wealth and influence, which is hinted to be vast enough to silence or make any investigator disappear, to manipulate events from the shadows.

The core of Tennōji's personality is a deep-seated disillusionment with humanity. He views the world as corrupted by endless conflict, crime, and environmental destruction, which he believes stems from humans becoming arrogant as the dominant species. This disappointment fuels his ultimate motivation: to annihilate existing humanity and create a new world order with himself as its absolute ruler. He is extremely manipulative and power-hungry, showing little regard for his subordinates or anyone else, whom he considers expendable assets in achieving his goal. His public persona as a responsible leader masks a cold, calculating, and ruthless nature.

In the story, Tennoji's role is that of the hidden puppeteer who rigged the modern-day Battle Fight. He deliberately keeps the Undead conflict active, using the Kamen Riders to seal specific monsters while secretly working on his true ambition. His key plan involves creating an artificial, all-powerful Undead named Kerberos by combining the DNA of all 53 Undead. To buy time for this research, he develops the Trial system and the Titan Undead as tools to challenge and occupy the Riders.

His key relationships are largely functional and antagonistic. He views the four main Kamen Riders as useful pawns to be discarded once they outlive their purpose, even attempting to have them eliminated. A nurse named Kotaro Shirai tries to investigate the connection between Tennōji and BOARD's secrets, but her efforts are suppressed by powerful forces loyal to him, highlighting his far-reaching control.

Throughout the series, Tennōji's development is a gradual unveiling from a background benefactor to the foreground villain. Upon revealing his true nature, he successfully creates and fuses with the Kerberos Undead, becoming the powerful Kerberos II. His arrogance and single-minded drive nearly lead him to victory. However, his eventual defeat comes not from the Riders alone but from the Giraffa Undead, Kanai. After Tennōji is defeated and separated from Kerberos II, he desperately pleads with the Riders to seal the Giraffa Undead (the Joker), as its survival threatens all of humanity. In a final ironic twist, Kanai kills the powerless Tennōji, exposing his ambition as that of a mortal man.

Regarding his abilities, Tennōji himself is a human who, in his final scheme, gains monstrous power by using a specialized card reader to transform into the artificial Undead Kerberos II. In this form, he wields immense strength, powerful enough to overwhelm Kamen Rider Chalice and require the combined efforts of all four Riders, including Kamen Rider Blade in his King Form, to defeat him.