Swimming out for almost five hundred meters, Li Yifei silently moved upwards, but he didn't fully surface, instead only exposing his mouth and nose above the water to rapidly breathe in a few gulps of air before sinking down again, not once allowing Aoki Reiko to break the surface.
Aoki Reiko had no idea that Li Yifei had gone up for air; all she felt was him swimming continuously under the water. When she became breathless, her only option was to seek air from Li Yifei's lips.
The first time, Li Yifei had actively given it to her; the second time, he did not offer, and Aoki Reiko, being a young woman, could not bring herself to actively take air from a man's mouth. But as the suffocating sensation intensified, eventually making her chest feel like it was about to explode, she had no choice but to go to Li Yifei for air.