[Mello being Mello: his reactions are difficult to hide. It was different when he had to put on the face for the mob, because that's over, and the fucker sitting across from him is directly responsible for his death.]

[No, he can't let that go. Probably never will.]

Help always comes with eventual payment.

[Again, bitter. Kira or no: this is a truth.]

As diplomatic as you seem, I have to ask: are you hoping to gain something from me once I develop my power?

[He means that too. Either Kira is watching out for how strong Mello becomes, or looking to use it to his advantage.]

I'll admit, I'm also seeking out fellow Witches for gain. I don't know what it all entails — [Perhaps the only truth he will reveal.] — but I do know that allies are beneficial down the line.

[So.]

What is it that you want, Light Yagami?

[No one just helps without a cause. He's sure of it.]