Bleach is a long-running shonen series that is beloved for its quirky humor, its huge cast of lovable characters, and one of the most beloved was also one of the first characters to appear: Rukia Kuchiki.
A capable Soul Reaper with deep knowledge and respect for the job, it was Rukia who first introduced fans and Ichigo alike to the ins and outs of the job. And, while it's true that Rukia contributed her fair share to the series' overall body count, defeating several mighty foes by its end, but her battle skills aren't what she's known and loved for. Rukia has a special place in the story, and that's why the fandom loves her so much. There is no Bleach without Rukia.
It's a stretch to say that Bleach is a true isekai series, but it does have some elements of the genre and its inverse, reverse-isekai, where an otherworldly stranger tries to fit into regular society. Rukia Kuchiki fits both molds, beginning with the reverse-isekai scenario. She was fun to watch in Bleach's first season because she had spent her life in rustic Soul Society, but now she goes to high school, sleeps in Ichigo's closet and argues with a stuffed animal in her day-to-day life. Rukia explores the mundane world of Karakura town with wide-eyed wonder as a visiting Soul Reaper, while also training Ichigo in the ways of the Soul Reapers, as she's in no position to do the fighting herself, what with Ichigo taking her powers.
Rukia shifts to the other side of the isekai paradigm in later arcs. She is taken back to the Soul Society for her crimes, reunites with Ichigo near the end of the Soul Society arc and makes peace with the Shiba family. During the Hueco Mundo arc, Rukia tagged along as a rather knowledgeable companion for Ichigo's team during the mission to rescue Orihime. Throughout the series, Rukia has been Ichigo's coach and ambassador to the world of Soul Reapers and Hollows, and fans love her for it.
Rukia is also beloved because of her personality. Rukia is all business when she's in a fight, like in her battles against D-Roy and Aaronerio Arurruerie, but she really shines during the character-driven moments in between the fights. Rukia has been through a lot -- poverty, hunger, loosing her original family...but she never let all that get to her. Instead, she became tough, kind and self-reliant, someone always willing to help. Rukia is a stern but well-meaning Soul Reaper coach to Ichigo, using her natural smarts and training to hone Ichigo into a fine fighter, and she has a personality to match. When Ichigo fell into a funk and refused to hunt Hollows for the time being, Rukia alone was able to shake him out of it and appeal to Ichigo's courage, pride and friendship to her. No one else could have done that, not even Orihime or Uryu.
Rukia's kind spirit also shines through, and she is generous and compassionate, despite all the horrors she has been through. Rukia never once takes out her frustration or grief on others; instead, she protects and supports Ichigo's sisters anytime they need her, and she deeply empathized with Orihime when the latter was held prisoner in Las Noches. Rukia was practically tripping over herself to apologize to Kukaku Shiba over her role in Kaien's death, and the guilt had been eating away at her. Even though Rukia had had a reason to kill him (a Hollow had possessed him), Rukia wouldn't rest until she made peace with Kukaku and Ganju, the late Kaien's surviving siblings.
Finally, Rukia did her part to fix her rather distant and cold relationship with Byakuya Kuchiki, with her earnest and heartfelt ways convincing Byakuya that she is a little sister to be proud of, not just a brat who needs some discipline. Ichigo also did his part to sway Byakuya, but ultimately, it was Rukia's good heart that convinced Byakuya to let go of his fixation on law and support his family, even if he must trample on every law and custom there is. Now that's some strength of character.