When people say "strong", do you mean like physically strong characters who can beat up bad people?
When I talk about "strong" characters, I talk about those whose ideals, even if represented minimally, has a major impact on either the story or a MC or other major characters. This is where All Might's sensei comes in. We know she was killed. We know she was not powerful as All Might. But just from the few things we do know of her about how she imparted the necessities of being a hero into All Might, how she told him to smile and taught him to show only that side to people he saves and what it takes to be a hero. All that alone, she is a stronger character than All Might even if her screen time was via some small flashbacks and we still haven't seen her face clearly. To me, personally, that's what a strong character should be.
That's why I consider Rukia to be strong. How she was still unwavering despite her powers being taken away from her, how she let Ichigo fight his demons (the grand fisher), how she believed in her friend and his ability after learning about not interfering the hard way (via Kaien's death), and everything ChocoBar9 said.
Nami is a strong character. Not because of her abilities or her sexiness or nonsense like that. Because of the Arlong arc. Where it showed her human side but also showed how much she endured as a character, as a human being and still put up a front and tried to survive in a world that let her down. These are strong characters.
Not some female character beating a big baddy. Sure, she might be a badass. But that's not a strong character.
