Rukia Kuchiki is arguably the strongest character in Bleach and here’s why.
She isn’t Ichigo. Let’s get this out of the way: her fighting skills are great, but no one can match up to a shonen protag in a fight. However, she is creative and strategizes well, making quick use of different people’s abilities to organize a decisive attack. It also helps that she’s a very skilled communicator. Not to mention the fact that her shikai has at least three different techniques, which is highly suggestive of the fact that she can probably develop bankai.
But that isn’t what makes Rukia Kuchiki strong.
Rukia Kuchiki is strong because she looks her demons in the eye. The death of Kaien Shiba shattered her, resulting in symptoms of PTSD and depression later on, especially during the Soul Society arc and the Hueco Mundo arc. As a result, she tends to blame herself for everything that goes wrong, from the death of Kaien itself to any danger Ichigo gets himself into (since without her, he wouldn’t be a Soul Reaper at all).
Almost everyone in her life has manipulated her or treated her coldly. Her adoptive brother and her childhood best friend dragged her back from the World of the Living for a trumped-up death penalty. No matter what he decided later, Byakuya Kuchiki coldly put aside any signs of affection toward her and put arbitrary laws ahead of her life. And as a result of her past traumas, she’s inclined to think that she is indebted to the Kuchiki clan, to the point where her life belongs to them. Her brother is, from the start, the very symbol of perfection, and his opinion is always right to her. Even if it means her death.
More under the cut—