id say Namine is a mix of Kairi and Ven
We've already discussed this and came to the conclusion this isn't the case.
It's the same reason as with Roxas, both their
appearances were affected by Ven's injured heart, not more.
Since both of them were born from Sora's body, in which Ven's injured heart sleeps, their looks were both affected by this.
@Mookystank:
Like Smile already pointed out, we aren't sure/ don't know which vessel Naminé had...as Sora's original body is with Roxas, like you said.
However, Naminé must have had some physical form or she wouldn't be able to experience physical sensations like i.e. Larxene slapping her around...
Sora himself, after being restored may have earned a new body shaped like his original one through Kairi's princess powers.
Execeptions wouldn't be called Exceptions if they would occur that often...

It was said in the Secret Ansem Reports in KH II clearly that Naminé is a very special and rare case.
Plus, as for the clustered hearts:
In the case of Sora and Kairi: Kairi's heart was within Sora's body, but unharmed and active, Ven's heart on the other side was fully merged into Sora's heart, badly injured and therefore dormant.
With Xehanort we have different circumstances:
When Eraqus "died" he probably hid his heart inside Terra's without him noticing (if Eraqus' heart is unharmed or not can't be said since we have no information about it).
Later, in the Keyblade Graveyard, Master Xehanort used his own heart to
invade Terra's and tried to subdue it.
So essentially these three hearts are close connected, while Kairi's heart was in Sora, but still independent...that's the difference.
EDIT: @Pratiko: Good point, for Naminé I would be tempted to say she's the exception of an exception, as you said, because of Kairi, she's even odder than Roxas.