Sen

was a mysterious monk who instructed Janis to follow Inuyasha and Kagome in feudal era. His true identity was Tsugumi, a human girl who gained Kamuitama from his husband, Datara. Under the influence of Gorai's Demon Blood Binding Spell, he was forced to listen to his instructions and lured Janis to unseal Datara's seal.

History
Sen's first appearance was at the moment Janis was attacked by a yōkai near Higurashi Wellhouse, when she intended to visit Kagome due to several days of absence in school. After Sen eliminated the yōkai and cured Janis' wound, he advised for to find Kagome by going through the Bone-Eater's Well and returned to Sengoku jidai. Before Janis left the modern era, Sen gave her two shikigami to help her reach her destination safely.

After Janis met Inuyasha and his friends in the feudal era, Sen appeared once again below the Goshinboku and met Janis. Sen mentioned that Janis' Kamuitama was revived after the Shikon shard fused into her body. At the exact moment, Inuyasha and Miroku showed themselves at there and cut off the conversation between them, forcing Sen had to left the place.

One day, Inuyasha with Janis and Kagome went back to the modern era and learnt that Umao and Shikako escaped, claimed that yōkai exist in Benten forest. When they entered Sen Shrine that situated in Benten forest, Sen mentioned once before that the forest was inhabited by evil yōkai and he sealed it to prevent people from getting harmed. Following Sen's order, Janis took off five seals from five small shrines that placed all over the forest and defeated the yōkai. When they returned to Sen Shrine and informed Sen, they saw a red glowing ball located behind Sen.

As Inuyasha and the others successfully arrived at Datara Shrine, Sen showed himself in front of them and gave another instruction to open the door that lead to Datara's place. Datara's servants Mitsurugi and Sugagaki opened the door to let them entered, later Janis tried to pull out the Arrow of Sealing from Datara's chest. At sudden the white light glowed in front of their sight. When Inuyasha and the others woke up from unconscious, they realised they been used by Sen and Gorai.

In order to defeat Gorai, who was now forced to follow Naraku's words, they tried to visit the Tempest Tower. Unfortunately, the Demon Blood Binding Spell within Sen's body obeyed Gorai's instruction and also gained immortality, which made it impossible to defeat Sen by using their powers. Unexpectedly, Kikyō appeared and gave a clue to let them find Sen's blood (that placed at Sen Shrine at modern era). Janis, Kagome and Inuyasha returned to modern era again to obtain Sen's blood. Then they went back to the feudal era to defeat Sen once again. Janis used her Kamuitama to bind his blood and his body together, and Sen turned into Tsugumi, who slowly awoke from the spell. Inuyasha and the others felt extremely surprised that Sen's real identity is Tsugumi, and Tsugumi started to explain the whole turn of events.

500 years ago after Tsugumi sealed Datara and killed her daughter during the wedding ceremony, Gorai put the Demon Blood Biding Spell onto her, made her Kamuitama tainted with evil demon blood and made her obey his words. She had waited this day that somebody like Janis would able to break the spell and regain her consciousness.

Since Inuyasha successfully defeated Tsugumi, she knew that her life-span was limited. At the same moment Tsugumi felt happy she could see her daughter (Janis) at her last breath.