temporary, interim, assumed (name), informal
假 centers on something that is not permanent or genuine: temporary, interim, assumed, or informal. It describes a stand-in or provisional state rather than the real or final thing.
假 combines 亻 (person) with 叚, which likely contributes the sound and carries a sense of borrowing or not being genuine. The character originally expressed the idea of something borrowed or provisional, extended to temporary or assumed states.
A person 亻 beside 叚, which looks like a borrowed or makeshift tool. Picture someone using a temporary stand-in while the real thing is unavailable.
For カ, imagine a temporary cardboard cutout standing in for the real person: cardboard -> カ, and the cutout is just an assumed, informal replacement.