[ honestly, he has no idea why he's asking her this - he doesn't know what answer he wants to hear. He's not sure he knows what his own happiness is anymore. Seeing his sister again, even if it was nothing more than the reflection in a mirror, made him happy, without a doubt, but she's something to work towards - a wish for the future. His present is happening now. Even if it's temporary. As Eichi said - aren't all things?

Suffering and despair don't bring him pleasure the way they used to. He knows it's not what his sister would have wanted; it's not the person he was, the one she loved. He wants to be someone she can love. He needs a different sort of happiness. He's happy when he's with Belph here. And... ]

... I liked talking to you at the party. It was almost like things were normal. And that day in the garden, too... [ a self-deprecating smile. ] Just for a little while, it was like I was alive.