Lux refers to his mother with the affectionate word okan, an antiquated term for "mother", while okan periodically calls Lux Yuki-chan, a nickname stemming from his birth name, Koyuki. When "strange men" visit their home, Lux can tell okan is nervous around them and he feels something of innate protectiveness for her. He knows better than to misbehave, even if only for his mother's sake, and obeys when she tells him to go outside, where he instead eavesdrops on the conversation out of concern. When he returns home after running away, okan scolds him for worrying her. She is later shown to be doting and comforting toward Lux after telling him they would be leaving their home.

Similarly to okan, Lux refers to his father with the affectionate word oton, the antiquated term for "father". Lux feels particularly safe in oton's presence, as shown by his relief when oton returns home while the "strange men" are still present. Oton would often tell Lux tall tales of his time in the Oyashima Armed Forces, describing countless great battles with ruthless dragons; young Lux always looked up to him thanks to these stories, and even aspired to slay dragons like oton once did. The night of Mizukyo's downfall, oton tells Lux—named Koyuki at the time—a bedtime story about a boy orphaned by dragons, of whom oton dubbed "Lux". This is where Lux would get his name from.

In the flaming wreckage of Mizukyo, an orphaned 8-year-old Lux is approached by Komainu, who offers him a choice: to train under his wing, or to die by his blade. Lux chooses the former, and thus is inadvertently "adopted" by Komainu, who renames him Wolf after comparing him to a wolf in sheep's clothing. In turn, Lux dubs him Komainu, the Japanese name for lion-dogs, reminiscent of his lion-like appearance.

Lux grows up traveling alongside Komainu, who teaches him how to slay dragons, among other things like the English language and, more subtly, how to suppress his trauma. Their relationship is shown to be strained, with Komainu acting overly aggressive and punishing Lux every time he slips up. Lux harbors his own hostilities in return, although he refrains from showing them, instead remaining respectful and obedient as was ingrained in him by Komainu.

Lux's submissive behavior finally reaches its breaking point when Komainu insists on putting Lux through a test and ties Lux up, leaving him to fight a starving doku no orochi by himself. He insults Komainu and lashes out in an attempt to defend himself, only to childishly plead for his adoptive father's return after being left alone to face the dragon.

When the doku no orochi initially attacks Lux, he nearly lets himself die. He thinks of himself as a burden to Komainu, who he feels sacrificed much to so pointlessly sustain Lux's life; feeling conflicted and frustrated by Komainu's decisions and his own failures to live up to them, he finally opts to fight back.

After Lux sustains injuries downing the doku no orochi, Komainu returns to help, only to yell at Lux for failing him in ways left unclear. Through an argument Lux learns the doku no orochi is Komainu's biological son, and becomes insulted by the notion Komainu cares more for the beast than he does Lux; Lux proceeds to go on a tangent about how Komainu is a terrible father. A disillusioned Komainu disowns him, and the emotional impact seemingly triggers Nox's first appearance in Lux'st psyche.

When the doku no orochi attacks Komainu and kills him instantly, Lux reacts viscerally, beheading the beast immediately after. Through a whirlwind of emotions he is shown to be thoroughly shaken by Komainu's death, crying out affectionate names and sobbing over his corpse.

Lux has conflicting thoughts regarding whether or not he considers Komainu family, and vice versa. Although he periodically shows varying levels of disgust and hatred for the man, he also admits to seeing Komainu as a father figure of whom he cared about. Lux is haunted by Komainu's death as well as his perceived failures as a son, and frequently thinks about his adoptive father with contradictory fondness and hatred. Lux can be seen discussing his ambivalence towards his and Komainu's relationship with Captain Emir Mardot; he confirms to her that Komainu took care of his biological needs, but also states the relationship felt mutually tolerant, if not hateful.

During his time as a bountier, Lux is commissioned by his client Jirou Yamabe to eliminate a chimera named Maria Whelan-Pollard. When the two meet in person at a bar to discuss the hit, Lux very quickly becomes irritated and impatient having to put up with Yamabe's overly lax, playful personality. Over the course of their little meeting, Lux is briefly amused by Yamabe's dramatics, although he continues to act very curt toward his client.

After Lux was captured by the Isaka Guild, on separate occasions both Soleil Rouxel and Captain Emir Mardot ask Lux for his last name, to which he gives his client's last name, Yamabe, in an impulsive attempt to protect his own identity. He quickly realizes he may have put Yamabe in danger and nearly outs his own real identity for the sake of his client's protection. Later on, when Captain Mardot makes mention of locating Yamabe, Lux is quick to ascertain the Guild did not harm the man.

When Lux returns to his hotel to collect his belongings, he notices several missed texts from Yamabe showing concern for Lux's whereabouts. Lux is somewhat amused by Yamabe's erratic texting habits, but soon feels guilty about giving Yamabe's name to Captain Mardot.

Lux and Maria first meet through rather harrowing circumstances: Maria was a target Lux was hired to eliminate. After weeks of gathering intel on her character, Lux attempts to herd her into an alley where he can kill her. She catches him off guard before he can secure the kill, only to dismiss him as a young and confused member of the Guild. He is made hesitant when Maria shows him mercy, but, believing her to be a dangerous monster, he attacks her anyway. During the fight, he insults her, and after gaining the upper the hand, he then hesitates a second time to kill her in light of her pleas to spare her alleged daughter.

At some point after both Lux and Maria are captured by the Isaka Guild, Lux wakes up in a glass confinement and finds he shares it with Maria. He immediately shows hostility towards her, blaming her for the destruction of the city and the events leading up to his capture, and makes empty threats on her life. Their conversation gradually loses tension, although Lux is persistent in accusing Maria of lying about him being a chimera. When he starts putting everything together and realizes her claim may be true, he begins to tear up and does not fight her gesture of comfort.

While Lux and Maria are both forced to share a living space, they soon begin to get along, asking each other questions and learning about their backgrounds. It is at this point Lux begins to feel a neutral acquaintanceship towards her as opposed to hostility, and he admits he finds her to be the most trustworthy person within the Guild thus far. He becomes a bit annoyed by Maria's growing penchant for teasing him, and periodically dishes it back.

After Lux transforms into a dragon for one of Elwyn's experiments, he struggles to maintain awareness, and with what little awareness he does have he notes he can use this dragon to destroy the chimera cage holding him and Maria hostage. He believes doing so would make up for him causing Maria to be captured by the Guild in the first place. He is stopped by Guild dragonslayers. However, in lamenting over Maria's forgiveness and kindness shown toward him, he is motivated to try again and even succeeds in breaking the glass cage.

When Maria transforms into her dragon to stop Lux, he misinterprets her intentions and feels betrayed by her actions, suspecting her of deception. He attacks her after being manipulated by Nox into doing so. Nox would later berate Lux for daring to feel kinship towards her.

Lux comes to after his dragon was defeated and briefly worries about Maria's safety, only to chastise himself for it. He is immensely relieved after spotting her alive and again tries to repress his feelings of friendship for her with the assumption that she probably despises him after he'd attacked her with his dragon. A group of dragonslayers passively let one of their own continue to harass Maria, and Lux demands they let him intervene if they won't do so themselves. Lux successfully intervenes, and is surprised when Maria acts happy to see him. He eventually asks her why she's not upset with him and is frustrated by her answer, practically begging her to hate him so it would be easier for him to forego any friendship between them, as per Nox's demands. He notes Maria is a lot like his mother, okan, when she comforts him. In his attempt to dismiss their friendship, he ends up apologizing to her for causing her to be caught by the Guild.

Due to the destruction of the chimera cage, Lux and Maria share temporarily living arrangements with Elwyn, and, implied by their lighthearted conversation, they have mutually reconciled their relationship. Lux is later shown to feel much sympathy for Maria after she learns she had inadvertently killed killing civilians, as he feels Maria is nothing more than a civilian herself.

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