Both Saji and Louise introduce as generic high-school student and both of them used to be classmate in Japan.
Louise and Saji had a pre-planned trip to HRL's True Pillar, a trip related to their aero-space studies. While Louise checked in the station for private-class transport, Saji's big sister, Kinue Crossroad, was having a concerned chat with Saji's relationship with Louise. Louise politely reassure to her to not worrying about them. Both of them become one of victim who trapped in True Pillar Incident that accidentally caused by Soma Peries. Luckily, they and the other innocents are saved by both Celestial Being and HRL Forces.
After the witness La Edenra bombing, Louise mother quickly come to Japan to pick her up for her safety. Louise refuse it and she demand to Saji to help her to reassure her mother that Saji is good boy. They successfully to use various way to convincing her, and Mrs. Halevy agree with Louise that Saji is a good person for her. After Mrs. Halevy return to Spain, she got depressed because she feel lonely, and Saji arranged Setsuna to help him to cheer him, and it's not successful due to Louise hate Setsuna statement about her parents. At dating, she interested with pair of gold ring, and demand Saji to buying them. After look the price, he begin working hard and save his money to give those ring. Not longer after that, at school break, Louise tell to Saji that she need to return to Spain to attend her cousin wedding. Before leave, she want Saji to kiss her at airport.
Unfortunately, after hearing that the place where The Halevy's attend the wedding is attacked by Nena Trinity, and leaving only Louise who survive the accident, Saji hurry to go to hospital to check her condition. Saji bring her a rings that she wanted, and Louise is happy with it. Sadly, she told to him that she unable to wear it because of her left is amputated, and revealing Louise become emotionally unstable. Several days later in hospital, Louise convinced him to return to Japan to continue his study and his life. She told him of her dream to go into space and that one day they'll be together above the orbital elevator. With regretful tears, she watched Saji leave back to Japan.
After Operation of Fallen Angels,
After the lose of Celestial Being, Saji continue his study and Louise still remains in hospital. At some point, Saji successfully achieve his dream to work at space, but Louise ended join A-Laws and become pawn of Innovator for sake to seek a revenge of her family.
Before their reunion, it's shown that both Saji and Louise miss each other and always wondering what happen to them. In episode 8, Setsuna meet Louise at federation banquet with Louise greet him. Louise still remember Setsuna and they begin have private talk about Saji. Setsuna lie to her that he share same workplace with Saji and tell to her that he is alright. Hearing that, Louise is happy and looked at the ring, tell to him that she glad that Saji finally achieve his dream. When Setsuna see strange mark on her arm, she told to him that it was caused by small accident and reassure him to not worry about her. Setsuna remember Saji words about her suffering, and he decide to not continue to ask. When Setsuna tell to her that Saji always thinking about her, she suddenly suffer headache. When Billy Katagiri see him and expose his identity as member of Celestial Being, Setsuna runaways and leaving Louise with Andrei to handling her.
In battle of Lagrange 3, they finally reunited in opposite side, and begin asking to each other why they are on battlefield. As the result, Louise mistaken Saji betray her, and resulting her mental condition become worsening.
On second time meeting with her, Saji and Louise have private conversation via Trans-Am burst, where they seen in Quantum Plane, looking on earth. Saji convince Louise to return with him, but she refuse. She treaten to him that she will shoot him if he dare to get close to her. Saji kept walking towards her and asked what had happened to the woman he once knew. Louise said that she had changed herself, but Saji wouldn't believe her and kept walking towards her. Eventually, he got to Louise and put his hand over the gun barrel, giving Louise a hug. In her fragile state, Louise relived the pain of losing her family and her hand, and is pulled out of the vision by Andrei. When Saji and Setsuna decide to find her once again, they were stopped by Mr. Bushido, who no other is Graham Aker, to rematch his fight with Setsuna. At same time, they see Louise with GNMA-Regnant brutally attacking Nena Throne Drei, tell to her to stop because revenge will never bring anything. Louise successfully kill Nena, but she has no satisfy with her action because her family still death, begin to swift her revenge on Celestial Being.
On her final showdown, Saji asked her if that would make her happy. He states that they were trying to make a world where everyone can understand each other. However, Louise blamed Celestial Being for distorting the world and causing war and continued her attack. She use suicide attack to kill both Saji and Setsuna. Fortunately, it was stopped after Louise get knocked by three GAGAs force. Louise is survive, and Saji pull her unconscious body, evacuate her at saved place. Unfortunately, when Louise regain her consciousness, her mind still corrupted by Ribbons influence and start to chokes Saji to revenge. She begin crying after see his ring and screaming in pain due to headache, and once again lose her consciousness. After Setsuna use quantum brainwaves to activated Trans-Am Burst System, the entire colony in a massive flow of GN particles, renewing Celestial Being's determination to fight on. Louise awakened and said that her heart was melting, and Saji said that the warmth was from Setsuna, whose light would lead them to the future.
With A-Laws and Innovator defeat, Louise decide to fully retired from military to curing her condition after what already Innovator had done to her, with Saji always be in her side.
- “You can't stay, Saji. You have to go home. Wait for me in space. . . I'll catch up to your soon”
- — Louise Halevy message to Saji about her dream and her promise
At the first time, SajiLouise not so popular among fans. After their romance play as main subplot of season 2, this pairing gain many fandom. Some of them are okay with them, but there are also a fandom who doesn't like them because of Louise behavior and for the secondary character, their romantic subplot are too forceful by making them into opposite side and fighting each others.
This ship is has rivalry with AndreiLouise , due to Andrei action try to separated them in various way even by tell to her to kill Saji.