"I just...well, I didn't want you to feel...uncomfortable like that," Hiro said. "It's...it's not anything weird. Really."
"Hiro-kun," replied Húxiān, her voice soft and sweet, "please don't worry about making me feel uncomfortable." Húxiān would move her chair closer to Hiro. "I trust you"---Húxiān would attempt to put her hand on Hiro's inner thigh in a gentle, almost platonic way---"and I like you." As Húxiān said those words, she would attempt to meet Hiro's gaze. Her hand would be resting precariously close to Hiro's groin---not close enough to necessarily be considered sexual but near enough that "Kimie's" intentions would be ambiguous. Furthermore, Húxiān's bare breasts would be tantalizingly exposed: all Hiro would have to do to catch a glimpse would be to let his gaze wander just a few inches downwards.
Then, Húxiān would gently ask, "Is something the matter? You seem very nervous. Does my lack of attire make you feel uncomfortable?"
Everything Húxiān did was designed to ratchet up the sexual tension while still maintaining plausible deniability. To force Hiro to question whether or not the seemingly innocent and earnest "Kimie" was actually coming onto him. And to establish, albeit abruptly, a sense of intimacy between "Kimie" and Hiro. Everything from referring to Hiro as "Hiro-kun", an endearment that implies a sense of familiarity, to placing her hand on Hiro's inner thigh and close to his groin, was designed to further those purposes. Finally, the line "and I like you" was purposefully ambiguous so as to keep Hiro guessing: "Kimie" could have meant that statement in a platonic way, or she could have been harboring romantic feelings for Hiro and simply have been playing coy.
Húxiān, on the other hand, only saw Hiro as a pathetic, spineless toy for her to play with---and a tool that she could use to take down a more dangerous target. Moreover, to Húxiān, even her own body was simply a tool to be used in pursuit of what she wanted. In Húxiān's mind, there was nothing special or intimate about nudity and sex: both could be used as weapons and the latter felt good physically. As such, Húxiān had no reservations about flaunting her body to Hiro.