all, together, accompany
俱 centers on the idea of being together or accompanying someone, often implying a group or all members involved.
俱 combines 亻 (person) with 具, which likely contributes both sound and the idea of gathering or being together.
A person 亻 beside 具 (tool/ingredient) suggests people gathering together with their tools, all accompanying each other.
For ク, imagine a group of people all together in a cool (クール) club, accompanying each other: cool -> ク.