As Merlin fixed his gaze on her, Luna didn't look away and nor did she change her expression at all. If there was one thing Luna was good at (one very dumb thing) it was pushing back against beings infinitely more powerful than her, whether it was a good idea or not.
Her expression only broke when he advanced on her, turning into a bright and delighted grin before he kissed her.
When Merlin broke away she ran her tongue over her lip and looked smug. "You'd be bored of me if I wasn't," she told him with confidence, before kissing him again, fierce and demanding.