In Season 1 most people consider the fact that Saito preferred Louise before she was under the influence of the Love Potion as an Idiot Ball moment, but think about it, he actually loves her the way she is.
Again in Season 1, Saito comes back to save the day, despite having no obligation to help fight.
Season 2 episode 4, when Saito confesses to Louise and they make out for several minutes in a boat.
The light novel's Volume 11's epilogue. Louise and Saito, after all the troubles they went through, reconcile and kiss; notable because of Louise (sort of) admitting her feelings for Saito directly to him, and him not screwing up his time alone with her, unlike in previous occasions.
Volume 13's Chapter 8. Louise asks Saito out for a date. Thinking it's a trap set up by Louise, Saito is suspicious of her behavior, but he agrees. They spend all day together, Saito noticing Louise's not so frequent smile, even kissing twice. Doubles as a Tear Jerker, as Louise, being told of the World Door to return Saito to his world, decided to spend her time together with him, thinking that day might be their last being together.
Volume 14's Chapter 9 and Epilogue. After Louise gets her memories about Saito erased by Tiffania due to him returning to his world (actually being sent 6000 years into the past by Julio) and her not wanting to suffer from his absence, Saito comes back to save her and their friends from Myoznitnirn's golems. After a while, Louise regains her memories due to Saito kissing her, and later they, alongside their friends, destroy the remaining golems. Later on, as everyone celebrates victory, Louise and Saito talk about his decision of coming back and their memories together. Turns into a Funny Moment as Louise sees a memory of Saito making fun of her while she was in a trance due to being under the effects of a crappy love potion, although he didn't know that fact.
Volume 18's Chapter 1. After Louise's return from Saint Margarita (where she fled to due to seeing Saito and Henrietta kiss) to save Saito from Jack of the Elemental Siblings, Louise and Saito reconcile after a long but heartfelt discussion.
Chapter 9 from the same volume. Eleonore, Louise's eldest sister, pays Louise and Saito a visit to their home. After Eleonore questions Louise's decision to live with Saito, Louise stands before her and says she'd rather lose her title as a noble than being separated from him. Eleonore was reluctant about Louise's decision but later accepts it. As Louise and Saito prepare the guest room for Eleonore, who ordered them to sleep in separate beds due to them not being married yet, Louise asks Saito where will he sleep. He replies he'll sleep on the couch, but she shows concern for his comfort, a sharp contrast to when they first met, where she ordered him to sleep on the floor, a fact Saito lampshades while receiving an apology from Louise. Later they spend a little time alone together on the couch.
The rescue and events after rescuing Tabitha that all of Saito and Louise's close friends agree with, even though Louise as well as her other Noble friends, including Saito decides to renounce their Noble statuses to do so against the Princess's request to not rescue Tabitha as it would complicate matters with the two conflicting countries with the danger of starting a war with them, along with Colbert acting as the decoy to fool the Princess and her forces, pretending that Saito and Louise are using Colbert's recently-made airship, the Ostrant, to escape the country of Tristain after the Queen decides to cordon it in order to prevent them from leaving, in order to infiltrate the country of Gallia where Tabitha is being held captive due to her failure of eliminating Saito, as she was pressured to do so as she is secretly a royal-turned-spy for the country of Gallia, with King Joseph threatening to kill her mother (which has unfortunately turned into a psychotic wreck years ago after protecting her daughter by drinking a poisoned drink meant for Tabitha) if she refused. As progress is made, King Gallia's personal elf guard is ordered to begin the process of brewing the very same dreaded potion that Tabitha's mother drank years ago that transformed her mental state erratically, as the elf is in the process of doing so he hands Tabitha a book, as she's in captivity, to read a story to pass the time, the elf says it's "Ivaldi's Tale" and says to read it while she is still sane before she is mentally doomed by said potion once it is fully brewed, and as she reads it, she mentions that she "betrayed" her "Ivaldi", and she whispers, with tears in her eyes, that no "Ivaldi" will save her... Saito, Louise and their company of friends disguise as a travelling circus troupe to infiltrate the country of Gallia easier. They decide to hold a dance festival hosted by the girls in scantily-clad outfits so they can put the Gallian soldiers to sleep with sleep medicine-laced drinks. After facing adversity of the King's personal Elf guard/potion maker, they managed to at last save and rescue Tabitha before the accursed poison is administerred. When they return to the country of Tristain after doing so they anticipate Princess Henrietta to punish them. She does... by gifting her with a royal mantle instead of her previous one and gives Saito his chevalier mantle back after they rescued Tabitha. As if that weren't enough, it turns out that in the chaos of the events before, they also managed to rescue Tabitha's mother who was also being held captive. Tabitha becomes absolutely infatuated with Saito as she does have a precious "Ivaldi" rescue her against her thoughts otherwise, crying tears of joy when she is rescued, and from thereon she likes to affectionately call Saito by "My Ivaldi" at times. Aww...
In Season 4, when an Ancient Dragon goes after Void mages, the place to fight it is decided to be the Academy of Magic. All students are evacuated... except for Louise's classmates — who decide to stay and fight the Eldritch Abomination alongside Louise!
Also, after Kirche learns of Tabitha's dark past and sees her haunted by nightmares in her sleep, she decides to lie beside Tabitha and embrace her, telling her everything will be alright.
Season 4 takes the cake. When the Ancient Dragon is about to devour Louise, Saito manages to arrive just in the nick of time to deliver a twin-missile strike from the F-16 Fighting Falcon fighter he pilots to the Ancient Dragon (which he had to fly through the supremely-rare "Eclipse" phenomenon in order to return to Halkenginia, with Derf returning via reviving inside his hand-rune to boot!) to distract it from accomplishing its goal, thereby being a an absolute shining moment of heroism that ends on an absolutely epic note, all while the "First Kiss" theme from the opening of season one beautifully plays in the background as both Louise and Saito put an end to the abomination. Saito proposes to Louise in front of everyone. Louise doesn't even try to hide her happinness and accepts it wholeheartedly. Then they get married. FOR REAL. Possibly in the same chapel where they got married in Season 2. With everyone (meaning the girls + their teachers + the royals + the elves + LOUISE'S FAMILY) attending and genuinely happy for the two lovebirds. Playing to the tune of 'I SAY YES (Wedding Version)' from Season 2's final episode. If that's not heartwarming enough, then let's go for the epilogue. Louise and Saito arrive at their newly rebuilt manor in Des Ornieres. Then they get off, Louise uses World Door, and she and Saito go to Japan. Louise experiences our world for the first time, including riding the bullet trains, sightseeing, and eating crepes. They finally reach Saito's house, where they push the doorbell hand-in-hand, their wedding rings glistening on their left hands' ring fingers and Saito's Gandalr runes still visible. Louise doesn't look annoyed, or angry, and she's really enjoying their precious time together. All this to the tune of Louise's happiest (or at least the only one where she doesn't snap at Saito) rendition of 'Kiss Shite Agenai'.
