Bro he literally told her the end of the last volume. 

Hot take: If it wasn't for Sunny's flaw being to keep him from lying he'd be the final villain Neph would have to fight before finally destroying the Forgotten God.

Think about it. If Neph wasn't thirsty for some Shadow Penis she'd have killed him to preserve humanity already. She'd hunt him down to the ends of the earth even more than Mordret and Skinwalker just to kill him. 

In the forgotten shore arc Neph copied Sunny's personality, especially how to lie and interact with people. That was only possible because Sunny could not lie. If he could lie Neph would've never been able to even guess what he's like and would've never gotten such a high opinion about him. That's where her feelings for him all started. 

It would've eventually lead to there being an insurmountable distance between them in their relationship created by a mountain of lies and deception. Sunny would essentially become what Asterion was to the rest of Broken Sword's cohort.

This was avoided due to his flaw