Because Kasumi was deceased right before the events of Persona 5 began, and thus was unable to speak for herself, most of her personality and history are only known by the accounts from Sumire's Confidant, as well as her behavior when she identifies herself as Kasumi.

Kasumi is largely implied to have the same personality as the "Kasumi" who is actually Sumire under Maruki's spell, aside that she is much more bold and confident and is fully consistent in her performances and cheerfulness