decoy, lure, stool pigeon
囮 centers on something used to attract and trap: a decoy, lure, or bait, including a person used as a stool pigeon.
囮 combines the enclosure 囗 with 化 (change, transform). The exact historical development is uncertain, but the structure likely suggests something transformed or disguised within an enclosure, fitting the decoy meaning.
Inside the enclosure 囗, something is changing (化) into a fake version of itself. Picture a box where a plain object is transformed into a tempting lure to trap the unwary.
For カ, imagine a clever crow (カラス) using a shiny lure to trick other birds: カ from カラス, and the crow's decoy sits inside the enclosure.