Namine is an interesting character. Based on KH lore, she shouldn’t really exist. Well, no Nobody should exist, but Namine isn’t even really a Nobody.
That's because Kairi has no darkness in her heart; the Ansem Reports even comment that Namine is a curious Nobody, especially given how she was formed.
Her existence adds a lot to the mystery to Nobodies and further suggests there’s more to them than just Heartless shells. More importantly, it makes Namine’s fusing with Kairi a bit confusing.
Kairi never lost part of herself, and Namine — though not whole — wasn’t lacking anything noticeable. Instead, she gained a substantial bit of power over her surroundings and Sora.
There’s always the possibility that Kairi harbors some sort of hidden darkness (despite being a Princess of Heart), suggested in Namine’s jealousy, and that’s how Namine was formed to begin with.
It wouldn’t disqualify Kairi from being one of the Light bearers (just look at Riku) and it would play into the theme of balancing light and dark.
But perhaps she’s simply a living metaphor for the personification of light, and that’s why she still interacts with her surroundings. Regardless, Namine must still have some work to do, else she wouldn’t feature on the box art as much.