Hakuren Oak is Teito's roommate at the church and, at first, his self-proclaimed rival for the priesthood exam.
They first meet soon after Mikage's death when Hakuren walks by Teito in the church. Teito mistakes him for Mikage since they look somewhat similar, but upon looking closer, realizes it is someone else. Upon seeing that Teito is training to become a bishop a well, he states that Teito is now his rival. Teito thinks there is something about Hakuren that is familiar to him until Hakuren asks Teito whether middle school children were starting to take the exam as well. Teito suddenly realizes that Hakuren sounds a lot like Shuri Oak, his tormentor from the military academy.
It turns out that Hakuren is related to Shuri Oak (they are cousins and Hakuren comes from the cadet branch) and he and Teito immediately despise each other, even thinking "if this wasn't a sanctuary, I would beat him up." After trading insults, Teito asks Hakuren why someone from the noble Oak family, having a reputation for being in the military and the government, would pursue a life at the church. Later in the day, upon hearing that they are roommates, both of them demand a change, but are rejected. Because they are roommates, they are also assigned as partners for training and the examination.
Eventually, they apologize for being so rude to each other and become friends. After Teito tells him about how Burupya is the reincarnation of his friend, Hakuren tells him about why he came to the church to become a bishop. When he was very young, his mother was taken by a Kor. Hakuren suggested asking the Church for help to exterminate the Kor, but his father said it could not be done because they had to protect the noble Oaks' good reputation. Her mother was shut in the house to keep the secret. Hakuren prayed for God's help and one day Frau came to his house and healed his mother. Hakuren believed that God answered his prayers and from then onwards he wanted to become just like Frau. After telling Teito his story, he felt embarrassed as he had never told anyone about that before, but they became good friends. Their friendship is further developed when a Wars attacks the two of them after training and Teito grabs Hakuren's hand trying to save him from falling. They both fell from the building but were saved by Frau.
After that incident, Hakuren offers Teito a handshake, calling him a 'comrade'. Teito hesitates, remembering how he became friends with Mikage but ended up being unable to protect him. He then tells Hakuren he will be friends with him when he's strong enough to protect him. Eventually, they become best friends, and before parting ways to begin their apprenticeship after completing the exam, promise each other that when they next meet they will be much stronger. He is now working as an apprentice bishop for Castor, sending Teito mail whenever he can.
Later on, he received a message from his father asking him to come back to the family to serve as a tutor to Princess Ouka, the empress-to-be. At first, Hakuren refused to accept the position, preferring to continue his training as a bishop, his excuse being that he dislikes women. When questioned about his feelings for his mother and Lazette, he states he views his mother as a mother, not a woman, and Lazette a child. However, Castor asks him to visit an old hospital with him before making his final decision. After seeing and hearing the things that were covered up by the military, he decided to comply with his father's will, to go to the empress-to-be in order to let the voices of the weak be heard by the royal family.
Hakuren (ハクレン) - is many mean in Japanese.
Oak (オーク) is many mean in Japanese.
- Hakuren and Princess Ouka go along with each other well, as they both like Teito.
- Hakuren was ranked fifth in the latest popularity poll in Monthly Comic Zero Sum.