After their first Wave fighting together, Malty reflects that she just might be happy living with Naofumi as a duchess rather than fighting for the throne.
Raphtalia and Malty both stick up for Rishia when one of Itsuki's other party members starts verbally abusing her.
Beloukas reveals that while his employees are his slaves, he's also theirs and that they're all a Family of Choice.
Naofumi's moping over the injuries the people of Lute suffered in the Second Wave is interrupted by Gaston who tells him he saved their lives. Port Harp was far better defended than Lute yet it was completely wiped out in the First Wave, which consisted of weaker monsters than the Second. Sure, Lute is damaged and several of the villagers are injured, some of them permanently crippled, but they're alive and can rebuild.
General
Naofumi's concern for his party members and Adorkable nature.
While Malty still holds anti-Demihuman views and sees Raphtalia as a budding love rival, it's kind of nice of her to calm the latter down when she thinks about her lost loved ones.
Later they get lost together in a cave, and Malty knowingly passes over a few chances to get rid of her without anyone knowing and helps her dealing with the situation. It's only after being put through hell that she panics in front of the two-headed dog monster... But everything she did before that has already ensured her Raphtalia's gratitude.
By the time Eclair joins the party, their relationship is so good and so close that the knight all but declares them adopted mother and daughter. And it turns out after the Third Wave that Raphtalia does indeed consider Malty a second mother.
Chapter 5/Interlude 1: The Melancholy of Naofumi Iwatani
Although Malty saved Naofumi from his False Rape Accusation by Pragmatic Villainy, Naofumi deep down was grateful for her actions and never became a cynical person.
Chapter 7/Chapter 6: Cloak and Dagger
It's admittedly endearing to know that Malty was going to help Naofumi from being attacked before learning that he was just acting out Captain America from a distance.
Chapter 19/Chapter 17: A Dark Descent
In a brief flashback, Malty's sheer horror when she discovered her sister was giving too much confidence to someone suspected to be a paedophile (suspected as in "everyone knows but nobody cares to actually confirm as long as he keeps himself to demihumans"). Because while she doesn't think he'd be stupid enough to actually try anything with a princess he could still be a bad influence, and it's better to be careful anyway. The fact he's actually Raphtalia's previous owner, who is stupid enough to do just that if given the chance, means Malty unknowingly saved Melty's life.
Chapter 26/Chapter 23: The Easy Way Out
Malty, questioned by Sieg as to what she even is if the throne is occupied by her sister and her own face is covered with scars, Malty's response is that her identity is to be Naofumi's hero.
After Naofumi's duel against Motoyasu, he frees his three slaves and comforts Raphtalia who thinks he's "throwing her away".
Chapter 52/Interlude 8: The Melancholy of Motoyasu
Motoyasu receives a You Are Better Than You Think You Are from both Naofumi and Bertia. As a result, Motoyasu acknowledges that Naofumi is a better hero than him, but lightheartedly warns him not to rest on his laurels or he'll overtake his rival.
Chapter 53/Chapter 45: Duty of a Hero
Motoyasu proposing his love for Iris.
Chapter 54/Chapter 46: Freestyle De-mask
the reveal that, in spite of their mother's attempt at ruining their relationship, Melty still looks up to her sister. Even Malty is moved by it, although she's not sure if it's because Melty looks up to her or because it means she's one upped their mother.
Chapter 91/Chapter 79: Order Tide
Malty's first thoughts after surviving a fish explosion from a wave monster is making sure to reunite with Raphtalia and Naofumi, because she doesn't want them to get endangered without her support. It really shows that Malty isn't willing to let any of them die and will risk her life for their safety.
Despite the rivalry and disdain Malty has for Melty, Malty shows her hint of care by demanding Miya to ensure Melty is protected from the third wave. Malty also gets to have more of her moral standards be shown, since as much as she may want to be queen, she wants to achieve it through her efforts as an adventurer and not because of her sister's death to leave her as the sole heir to the throne.
Chapter 94/Chapter 82: Hour of The Wolf
Raphtalia reacts to the goddess (her own web novel counterpart) trying to murder Malty by repeatedly coming between them, shouting she won't let the Wave take another mother from her.
