Onryo are spirit-like entities who take the role of causing mischief in daily life from normal Yo-kai. Most of them are involved in mysteries, and a select few can induce the transformation sequence of a Yo-kai Hero by being nearby.
A recurring plot element is them entering a second form, usually via Haruhiko Usmizawa's hair and special breath or Nayu Kuruhoshi's abilities. They are summoned via silver-framed Y Medals reminiscent of the original medals. Whenever their purpose is fulfilled, they disappear into the light and leave behind Y Medals. According to Goromi, Onryo are a level below Yo-kai in power.
List of Onryo
Onryo is based on the same subgenre of Yo-kai by the same name. In Japanese traditional beliefs and literature, Onryō (怨霊, literally "vengeful spirit", sometimes rendered "wrathful spirit") refers to a ghost (Yūrei, Japanese term for ghost) believed to be capable of causing harm in the world of the living, injuring or killing enemies, or even causing natural disasters to exact vengeance to redress the wrongs it received while alive, then taking their spirits from their dying bodies.
When their summon medal is used with the YSP Watch, a japanese announcement is heard ♪Come on, Ghost!♪ (Japanese: ♪カモンゴースト！♪ ♪Kamongōsuto!♪) after the initial Y, followed by lively electric guitar music.
YSP Watch Zero
When their summon medal is used with the YSP Watch Zero, a japanese announcement is heard ♪Summon! Yo! Yo! Ha! Ha! Ha! Ha!♪ (Japanese: ♪召喚！ よ！ よ！ ハ！ ハ！ ハ！ ハ！♪ ♪Shōkan! Yo! Yo! Ha! Ha! Ha! Ha!♪) after the initial Y, followed by lively japanese music.
YSP Watch Viper Connector
When their summon medal is used with the YSP Watch Viper Connector, a japanese announcement is heard ♪Snake! Come on, Ghost!♪ (Japanese: ♪スネーク！ カモンゴースト！♪ ♪Sunēku! Kamongōsuto!♪) after the initial Y, followed by lively heavy metal electric guitar music.
When their summon medal is used with the UR Watch, a japanese announcement is heard ♪Ultimate! Re-Re-Re-Revolution, Ghost!♪ (Japanese: ♪アルティメット！ レ-レ-レ-レボリューション！ カモンゴースト！♪ ♪Arutimetto! Re-Re-Re-Reboryūshon, gōsuto!♪)