Watch TDoS again, because the bulk of Sigma's conversation with Cain is about how X's worrying ties into his potential.
I'm not sure if I would call X's character "static", given that he continually questions his own actions and those of his enemies, making efforts to understand them rather than simply dismiss them.
But the trait of evolutionary potential is not just about X personally, but also how X affects the world around him. As Sigma put it, "the potential to advance all Reploids." It basically boils down to X's thought processes not being fully understood. X's worrying gives him a unique viewpoint that no other Reploid has. He can connect with others, be they human or Reploid, better than any other Reploid can, which leaves him in a position to influence the world in ways that other Reploids cannot, and to facilitate the connection between humans and robots. As long as X remains unique in that regard, he alone advances that relationship.
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