手柄話 – てがらばなし – boastful story
It seemed like Ryousuke was relating this episode as a boastful story. (Awkward translation…)
かきあげる – to comb upwards
He had a habit of combing his hair back with both hands as he talked.
跨る – またがる – to straddle
Of the remaining three people, two were sat on a bench and the other was straddling his bike.
ふてぶてしい – brazen, impudent, audacious
It was when it seemed like Yamashita was thinking of some further boastful thing to say, he combed back his fringe as was his way, and his mouth started to move. (awkward translation but nevermind, maybe it would be better as two sentences?)
呆然 – ぼうぜん – be dumbstruck, taken aback
In a daze, they watched the scene as though it were in slow motion.
直感的 – ちょっかんてき – intuitively, instinctively
At the same time, he instinctively felt he was in danger.