In Tite Kubo's Bleach, Ichigo Kurosaki has many important relationships, and these relationships are his whole reason for fighting and struggling as a Soul Reaper. But his relationship to Orihime Inoue may be the most important of all.
Ichigo and Orihime always understood and accepted one another, as opposed to the strange newcomer Rukia who butted heads nonstop with Ichigo. Orihime is one of the most important people in Ichigo's life, and without her, he would not have gotten very far in the battlefield or in matters of the heart.
Hero For A Hero
The first few arcs of Bleach make it clear that Orihime and Rukia are the two most important women in Ichigo's life, and in fairly similar ways. While it was Rukia who granted Ichigo Soul Reaper powers and trained him at first, it was Orihime who stood by his side during the Soul Society arc to rescue Rukia and restore the circle of friends. Rukia had been powerless ever since she gave Ichigo her strength, but by contrast, Orihime gained new powers, based on her hair clips. They could manifest as six fairies with unique abilities, and Orihime began to use them in battles against Hollows both alone and to support her friends. More than once, Orihime showed incredible courage and loyalty, fighting alongside her friends despite being an otherwise ordinary human being with no training.
Orihime became a critical member of Ichigo's squad in the Soul Society arc, demonstrating her incredible ability as both a healer and defender. She and Ichigo trusted and respected one another, helping spur Ichigo on as an action hero. She also inspired Uryu Ishida, and he was just as driven as Ichigo was to keep Orihime safe.
Later, during the Arrancar arc and the invasion of Hueco Mundo, it was Orihime who needed rescuing, but even then, she was Ichigo's ally and inspiration the entire time. Ichigo lost his life to Ulquiorra in their first fight, and it was Orihime who healed him, and it was she who restored Ichigo's spirits in the final battle with Grimmjow. Ichigo wasn't motivated to defeat his rival Grimmjow out of spite or even to defeat Sosuke Aizen. He was here to rescue his friends and justify their faith in him, and the bond he had with Orihime was worth taking on the world for. Orihime was as much Ichigo's hero as Ichigo was hers, both from her combat abilities and the strength of spirit and trust that she represented. Orihime embodied the best of what Ichigo's side was fighting for, and without her, despair may have won.
Matters Of The Heart
Rukia was the reason for Ichigo's career as a substitute Soul Reaper, but Orihime was the reason Ichigo kept fighting. Ichigo was never really aiming to save the world or earn the admiration and respect of the Soul Reapers. He did accomplish those things, but as a byproduct of his real mission: to protect the people close to him, and to create a world where they could live in safety and peace. Most of all, he did that for Orihime Inoue, and she wanted something similar for Ichigo. After her brother Sora's passing, Orihime realized that Ichigo Kurosaki was the most important person in the world to her, and she couldn't keep a lid on those feelings forever. On the night she was abducted to Hueco Mundo, Orihime visited Ichigo's bedroom at night and healed him, then gathered the courage to admit a few things out loud to herself. She was going to miss Karakura Town and all her favorite foods, and she had wanted to pursue all kinds of careers, from a baker to an astronaut. But most of all, she would miss the young man she fell in love with, and with great reluctance, left his room to meet with Ulquiorra. Ichigo awoke and realized at once what had happened, and it galvanized his will to rescue his friend no matter the odds (or consequences).
By the time of Yhwach's defeat and the fall of the Wandenreich, Ichigo had created his peaceful world at last, and he retired from the substitute Soul Reaper life at the first opportunity. He didn't define himself through combat as Kenpachi or Ikkaku did; he married the love of his life, Orihime, thus symbolizing what all this fighting, pain and bloodshed was all for. Fighting isn't fun; it's done out of necessity so people like Ichigo can spend the rest of their days with the people who matter most to them, and create something for the future. In this case, he and Orihime's son, Kazui. Ichigo's relationship to Orihime is why he fought throughout Bleach, and his marriage to her proved that his real mission was a resounding success.