Ken knew he couldn't dodge such a slap from a friend as Jane just did.
Unless he actively sensed the air and Qi around him as he did while traveling outside, he would be unable to sense a sneak attack from someone he trusted not to attack him.
He might be able to react on time if the attacker revealed a hint of killing intent, but Diana didn't exude any of that; she only meant to slap her friend for teasing her. Since he didn't believe they were stronger than him, it meant they were simply in sync with each other and their bond was impressively strong.
"Haha, see? This is why she has this nickname!"
Jane giggled as she took a step away from Diana's violent tendencies. Ken laughed with the tanned girl, agitating Diana further.
"Hey, your nickname isn't much better, Swordsman!"
Ken looked at Jane with curiosity, but she just shrugged it off with an unbothered expression on her face.
"It's because they say I'm a bit of a tomboy."