The way Kubo handled Nel's character arc is actually something I've gone back and forth on over the years. Her introduction as a child-like Arrancar in Hueco Mundo completely masked her true power and rank as the former Tres Espada. That first transformation during the battle against Nnoitra was pure spectacle—seeing this tiny, fragile-looking figure erupt into this graceful, deadly adult form with that iconic helmet and lance was a jaw-dropper. The design shift isn't just cosmetic; it visually communicates a total change in demeanor, from a playful, protective child to a seasoned, weary warrior.
What I find more compelling, though, is the psychological weight of her power. Her adult form represents her past and the responsibilities she abandoned, while her child form is almost a subconscious regression, a escape from the brutality of the Espada hierarchy. The manga doesn't spell it out with endless flashbacks, but you can piece together her trauma from her reluctance to transform and her affection for Ichigo's group. Her powers—Cero Doble, that incredible regenerative capacity even in her child state, her speed—all speak to a warrior of immense latent strength who chose to suppress it. It's a quieter, more poignant exploration of power compared to, say, Ichigo's constant new forms, rooted in themes of identity, memory, and self-preservation.
That final scene where she transforms one last time to save the day and then reverts, exhausted, really drives home that her power is a double-edged sword. It's a part of her she can't fully embrace without cost.