Rien has golden eyes and violent hair with a large single bang hanging from the left side of his face. He wears a white robe, a matching long-sleeved hanfu, a hakama, and black boots.
Rien is mainly calm and focuses solely on his research on concocting an Elixir of Life, which is the very reason why he was created in the first place. He takes precautions if anything were to interfere with his plans, warning the Tensens to not underestimate the current human travelers and sending out the Dōshi to inspect them. Like the other Tensens, he shows no sympathy for humans, viewing them as merely lab rats for his research and as subjects to create more Tan.
He cares a great deal for the Tensens, viewing them as family and states to them that they should treat each other as such. After seeing Mu Dan's corpse, Rien went out of his way to leave the palace to search for his killer and expressed his vengeful attitude to Chōbē asking if he was the one who killed Mu Dan. Even though he loves his family, he does not tolerate insubordination and will not hesitate in punishing them, as with the case of Mei in the past. According to Tao Fa, Rien is viewed as being frightening if any of the Tensens were to commit an action that he disapproves of. This causes them to know their place and not think about defying his authority. After Ju Fa blows up Zhu Jin's head in anger for failing his duties, Rien warns him angrily not to do it again, which the latter agreed.
Rien is deeply loyal to his creator Xu Fu. He continued on his work for the past 1,000 years and after finally presenting him the completed elixir, he bowed in respect.
1,000 years ago on Kotaku, Rien and Mei were created by Xu Fu and became his first disciples. They then took over his research in finding methods in granting immortality through the Inner and Outer Ways of Tan. However, Rien underwent a change having found the Outer of Tao to be more effective and killed many in the process. After Mei tried to stop him from going further, Rien punishes her by breaking her tanden, reducing her body to a child's, and sentences her to engage in Bōchū Jutsu with the Dōshi. He then becomes the leader of the other Tensens and formed the group known as Lord Tensen where they continue on with Xu Fu's research.
Lord Tensen Arc
Rien gathers the other Tensens for a meeting to discuss matters about the humans who have been causing a lot of trouble since their arrival. After reviewing that Ju Fa and Zhu Jin have confirmed the deaths of their victims, Rien closes the meeting with a toast to Soshi and their longevity.
Later, Rien gives a lesson to one of the Dōshi about Tao and is interrupted by Tao Fa who shows him the modifications she has done to the training area. After finishing Bochu Jutsu training with Ran, Ju Fa asks Rien the whereabouts of Mu Dan who answers that he is at the gate waiting for the human travelers.
The next day, Rien finds Mu Dan's corpse and leaves the area. After entering the Sōshin Nest, Rien finds Aza Chōbē and questions if he was the one who killed Mu Dan. Rien unleashes his Tao on Chōbē and becomes impressed to see that he has achieved hybridization after being infected by the Waitanhua flower down at the pit. He then examines Chōbē further and sees him enter into his transformation. Rien is overwhelmed by Chōbē and gets his body destroyed after Chōbē pierces his tanden. However, another Rien steps out behind Chōbē and reveals that he has killed one out of the many clones he is with. Rien smiles in glee after Chōbē demonstrated his control over the Tao of the Waitanhua flower, stating that he was "beautiful". He then knocks him unconscious and takes him back to the palace.
After Chōbē wakes up restrained to a bed, Rien presents himself in their Yin form and tells Chōbē that his Tao is extremely useful in finally completing her research. She then undresses and wishes to engage in Bōchū Jutsu to help his Tao cycle and better understand it. Before Chōbē could cooperate, he demands that she tells him everything about the island. Rien explains to him that the island is a laboratory meant to create and observe the immortal life of the monsters and the people known as Hōko. She also explains how the Tensens came to be and why she finds Chōbē's Tao invaluable so that Lord Tensen can perfect the elixir and turn every human on the mainland into Tan. Rather than turning him into Tan as well, Rien offers Chōbē a chance to join them if he cooperates. Chōbē accepts and the two proceed with their engagement in Bōchū Jutsu.
Palace Invasion Arc
Rien, and the other members of Lord Tensen, preform a Dōshi ritual with Chōbē participating. After they were finish, Rien sees Ju Fa starting a fight with Chōbē and orders him to stand down. Rien then has Chōbē follow him into his laboratory and tells him about the changing layout of the palace. Once they reach the laboratory, Rien shows Chōbē the Banko, a giant Waitanhua meant to process greater Tan, and requires his rare Tao to complete his research using his limbs that he plans to pick a part.
During the experiment, Chōbē tells Rien of the possibility that the survivors of the vanguard party will invade the palace. Rien then sees the opportunity to use the first group for the Rite of Just Consumption while holding off the second group. Chōbē then makes an agreement with Rien to cooperate as long as they spare his brother Tōma.
While the Tensens engage the humans for the ritual, Rien reviews the situation. He then presents Xu Fu the completed Elixir of Life and announces that they are ready to depart to the mainland to create more of the elixir from more humans.
Abilities and Powers
Rien is the most powerful member in Lord Tensen. He has full authority over the six Tensens, the Dōshi, and the other monsters that roam the island. During his confrontation with Chōbē, Rien was able to easily knock out the bandit in his berserk transformation, which was enough to cause Gabimaru a lot of trouble and almost made him go on to the verge of death, after finish analyzing him with one of his clones.
Through unknown means, Rien is able to create multiple duplicates of himself to act as his stand in's in order to make inspections. The clones are able to use Tao, however, they do not possess Rien's regenerative abilities. Also, they have been noted to be less stronger than the original but are nevertheless capable of handling strong opponents, as one was enough to push Chōbē into entering his berserk transformation.
- Further information: Tao
Rien possesses Earth-attributed Tao and has complete mastery over it. After meeting the remaining six Tensens, Chōbē sensed that Rien's Tao reserves is much larger and far superior than that of the other Tensens. His control was greatly demonstrated when he was able to cause damage to Chōbē's body after releasing a wave of Tao from a mere flick of his finger and easily defeated him with a single Tao palm strike. He can also gather up his Tao in a single highly concentrated ball of energy that can cause massive destruction.
- Distant Strike (遠当 Tōate?): Rien can launch his Tao from a distance as invisible projectiles, which are strong enough to cause major damage from within the targets body.
As a Tensen and a consumer of Tan, Rien possesses regenerative abilities and immortality to a certain degree. Rien's regeneration can only be nullified for a short amount of time if he faces an opponent that possesses the Wood-attribute of Tao and the only known way of killing him is if the said Wood-attribute user completely destroys his tanden, which acts as his source of power.
Rien is able to switch chi's of either Yin or Yang to augment his Tao and recover any lost. As Rien switches in and out, their sex changes to match the chi, which is put to use whenever they engage in Bōchū Jutsu training.
Being one of the first disciples of Xu Fu, Rien was taught everything his master knew. He possesses great knowledge of Taoism and the life energy known as Tao. Using this knowledge, he is able to impart his wisdom over to the Dōshi and conduct experiments that lead him closer to creating an Elixir of Life, such as the prototype known as Tan and the Banko.
- Rien ranked 33rd place in the manga 1st Character Popularity Poll.
- Rien can be seen as being the strongest Tao user in the Jigokuraku series, seeing as how his reserves are larger than the Tensens under him who have crafted their Tao for 1,000 years that go beyond human limitations.