Naofumi Iwatani was one of the people summoned to the kingdom of Melromarc to be the hero who saved the world. However, rather than being armed with something cool like swords or spears, he is destined to be the shield hero. And it doesn't take a long time for Naofumi to realize that he is not welcomed in that country.
Naofumi was scammed by his first companion, and even falsely accused of rape. He was thrown out of the kingdom penniless and in disgrace. Nobody believes in him and everybody treats him like some kind of criminal. Nevertheless, ignoring all the jeers and curses, he soldiers on and keeps on fighting monsters every day in order to increase his strength. That being said, it's not all bad though, along the way, he will encounter numerous trusted comrades who are willing to fight together with him. One of which is a girl named Raphtalia. That being said, the relationship between Naofumi and Raphtalia is actually much more complex than you might think.
Master and Slave
As the shield Hero, Naofumi cannot use any other weapon but his shield. He can't use a sword, a bow, or even a tree branch as a weapon to complement his shield. So in order to solve that terrible predicament, Noaofumi has to find another person who is willing to fight together and be his sword.
Unfortunately, Naofumi has a rather horrible reputation in Melromarc, which means nobody wants to fight together with him. Feeling desperate, Naofumi has no other choice but to visit a slave trader and purchase a slave that can be his sword during a fight. His choice falls upon a small demi-human girl named Raphtalia. So begin their relationship as a master and slave. However, unlike her previous masters, Naofumi never raises his hands on Raphtalia. On the contrary, he gives her medicines when she is sick, lets her eat her favorite foods, and buys some decent clothes for her. Yes, she might be a slave, but for Raphtalia, life with Naofumi is the best thing that ever happened to her in years.
Sword and Shield
Raphtalia knows that her duty is to be the main attacking force for Naofumi. In a sense, she is Naofumi's sword. As such, she diligently practices using the sword every day and is always eager to hunt monsters together with Naofumi. As their level increases, so does her sword skills. Raphtalia used to suffer a massive panic attack whenever she saw a big, violent monster. After all, the reason why she becomes a slave in the first place is because hordes of monsters went on a rampage on her village and killed her parents.
Naofumi is the one that dispels the fear and trauma from Raphtalia's heart. Her gratitude and admiration towards Naofumi filled her with the courage to charge and strike every enemy that came their way. Likewise, Naofumi also grows to be more attached to Raphtalia, and he wants to do everything he can to protect her, which allows his shield to be even sturdier and more dependable than before. They truly have become each other's sword and shield.
Man and Woman
At first, Naofumi treats Raphtalia like a daughter. After all, she was just a little girl when he first bought her. However, demi-humans grow in different ways than regular humans. They don't grow according to the years, but rather according to their level. The higher their level, the faster their body grows. Since they've been doing nothing but fighting monsters every day, it is understandable that their level will also increase at a faster rate than normal. As such, it doesn't take a long time for Raphtalia to grow into a beautiful young lady.
However, it is not her appearance that finally makes Naofumi see her as a proper adult woman. It is her unconditional support, unreserved kindness, and her unconstrained affection that ultimately open up his heart. She is the pillar of support that always helps Naofumi when nobody wants anything to do with him. That being said, none of them has openly expressed their feelings to each other. But it is clear at the latter part of the story that both Naofumi and Raphtalia wish for their relationship to evolve into something more.
Interdependent
According to the original novels, at some point in the latter part of the story, Naofumi will achieve the power of absolute defense through his shield in exchange for zero offense. On the contrary, Raphtalia will also obtain the power of absolute offense in exchange for zero defensive ability. These peculiar powers take their original traits to the extreme and basically make them interdependent of each other. Sure, Naofumi can protect anyone from anything with his absolute defensive power. Likewise, Raphtalia can also attack any enemies that come her way without any need for a defense.
However, if they are fighting against opponents who hold the same level of power as themselves, then it is necessary for them to combine their power. After all, Naofumi is the only one that can provide the absolute defense needed by Raphtalia so that she can always go all out with her attack. In a sense, Naofumi and Raphtalia are the only ones that can help each other reach their full potential. As you can see, the relationship between Naofumi and Raphtalia takes on numerous forms throughout the course of the story. However, slowly but surely, their relationship progress from a simple master and slave, to become the only ones who can complete each other.