Ki Tanmoku bears a striking resemblance to his own ancestor Rakugetsu.
Ki is referred to as a "handsome Korean- actor look-alike" by Keika You in the anime. His typical attire consists of a long black hoodie, white button-up shirt underneath, black tie, and black jeans, topped with a blue and black striped scarf draped over his shoulders.
He has white hair with white eyes in the manga and brown eyes in the anime. His hair is kept short with two long bangs framing the sides of his face.
In the manga, Ki is often shown with a cigarette and tends to smoke most of the time but this was taken away in the anime.
Ki Tanmoku is shown to be indifferent and cold around most people barring his master and Keika You.
He often shows a caring and protective side to Keika. Ki's cousin Jimei also points out that he has only ever seen the man smilingly blissfully in front of Keika.
However he can also appear to be quite heartless as shown when he fired Tanmoku's family's butler, who has served them for a long time, after his son almost killed Keika even though the butler begged him to forgive both his son and himself for the idiotic acts caused.
According to Keika e is also quick-witted and quick to solve problems. This was shown when Jimei and Jiun along with the Shito family blamed Keika for using the Rakugetsu sword without permission to hurt Ritsu Shito. He silenced all of them when he gave the sword to Keika to use it as his own.
He seems stern, stoic and mature above his age due to the role he has to play as the master of the Tanmoku household.
He is also shown as a sleepy head who is always sleeping if not eating. This seems to irritate Keika as the former does not tend to 'get fat' even with his lazy, inactive schedule.
You Keika - You Keika is the spirit shadow/ soul image of Tanmoku Ki. He was chosen when he was hit by a truck and died on the spot. Tanmoku Ki convinced Keika to be his spirit shadow when he the told the latter that he will disappear within the seven days of his death or be turned into an evil spirit.
Ki is shown to have a very gentle and caring side to Keika, often saying that he is the one thing that is important in Ki's life and the precious person he holds dear. He was visibly worried and unhesitatingly showed it when Keika told Ki that his soul was almost ripped apart by Shito Ritsu. Ki yelled at him and told him that he should have called him. When an irritated Keika yelled back that he called for Ki several times in his head, Ki was flustered and even blushed at the fact.
Ki is able to sympathize with Keika at the fact that he knows what losing parents feel like and stated that he wanted to be Keika's family. He was also the one that paid for Keika's funeral.
Ki goes to great lengths to protect Keika. Giving him a robe to protect him from other powerful spirit shadows and sacrificing his own life to safe his 'soul image'. He also cares a lot about Keika's feelings and emotions to the point where, while he was wounded and healing, he drove Keika to his home town on the seventh day of Keika's death, so that the boy could express his feelings and visit his friends one last time.
There seems to be a romantic development between the two protagonists. This was shown several times over the story. It was considerably depicted when Ki informed him, while they hugged, that the braid that was specially made for him, tied in his hair, lets him live even if Ki dies and kisses his soul ring to release him of his "distance restriction". To save Keika's life Ki casts him away to a safer distance. When Keika returns to save Ki but loses his soul instead, Ki calls out to him desperately. When Keika doesn't respond and is indeed shown to be soul-less, Ki is considerably saddened and loses his will to fight.
In episode 10, when Ki calls Keika to his room, a wounded Ki was being told by Keika that he should go and mend the wound, Ki leans in and kisses Keika on the lips. Keika blushes and asks what that was all about but Ki faints on top of him before he could give an answer.
Ki and Keika's relationship is a running gag in both anime and manga as a "married couple" because of the soul contract that binds them both together for life. The ceremony for the contract is even similar to a wedding where they pledge their vows to each other.
Mistress- The mistress is the grandmother of Ki and the current female head of Tanmoku household. She deeply cares for Ki. When she was informed that Ki was stabbed and was in danger she immediately fainted at the news.
Jimei Tanmoku- Jimei is his cousin and his personal secretary. They have a slightly complicated relationship where even though he takes care of Ki properly and effectively he is still reluctant at accepting him as the Yomeishi .
Jiun Tanmoku- Jiun is his cousin. Even though they belong to the same family their relationship is quite complicated as they both don't share a positive outlook on each other.
Master- Ki has a master that taught him many things he knows presently including some martial arts and sword fighting. He turns to his master for help when he loses his spiritual powers.
Ritsu Shito- Ritsu Shito is the main villain in the anime and tries to hurt Ki and Keika multiple times. He bares a strong grudge towards Keika for having killed his parents.