Love
The transformative capacity of love borders on the supernatural. It converts misers into philanthropists, cowards into heroes, and rational adults into babbling fools who speak in third person about their beloved's digestive habits. Love has driven individuals to cross oceans, scale mountains, and endure unspeakable hardships with a smile. The chemical cocktail it releases—dopamine, oxytocin, serotonin—literally rewires neural pathways, creating measurable changes in brain structure. Few forces in nature or culture possess such power to remake the human animal.
Sherlock Holmes
Holmes transforms those around him in more modest but measurable ways. Dr. Watson evolves from a wounded veteran into an accomplished chronicler; countless readers have developed improved observational skills from studying his methods. The character inspired real forensic science advances, including the adoption of fingerprint analysis. Holmes transforms by teaching—a noble enterprise, certainly, but operating within the bounds of the already-possible rather than remaking human nature itself.