That scene in the OVA was weird. Kazuma admitted that he treats Megumin, Wiz, Darkness, and Yunyun as walking sex toys but he lied to Aqua.>I just can't see you as a heroine
No he was thinking about telling her about the weird stuff he did to her when they slept together but he didn't. Also notice how he shakes his head after smelling her hair. Yeah Kazuma is a perverted scumbag but he won't go beyond the boundaries he set for himself.
Also he was making the other 4 girls do sexual acts. Only Aqua was exempt from these duties and instead ran errands. I feel like the OVA was trying to portray an important aspect on the relationship shared by Aqua and Kazuma.>Aqua knew Kazuma was jerking off next to her
We know Aqua is a punk. She's not the type of person to just let this action go without laughs. Normally we would expect her to taunt Kazuma on his downfalls. Instead, she chose to tell him about this when they were privately exploring a dungeon. There's clearly a line that Aqua and Kazuma won't cross.
Also in Light Novel 3 epilogue we got to see what goes on in Aqua's mind.>Aqua thought Kazuma was the one causing problems>she thought of the other 3 as children>she pities Kazuma and thinks the other 2 girls tend to give him a hard time>she wants to treat Kazuma better>tl:dr the two won't go beyond playful banter because they value the relationship they have