Kasumi Nakano is the daughter of Kenji Nakano and Rei Nakano. She and her grandfather used their mechanical knowledge to restore the pre-War appliances around her house. After restoring a radio and contacting DiMA, she is led to believe that she is a synth. This is reinforced by her inexplicable odd dreams and innate mechanical ability. She later flees to Acadia, although she can be convinced to return after completing the add-on. If Far Harbor, the Brotherhood of Steel, or the Institute attack Acadia, she will die in its defense.
She is curious and naive, as shown by her pursuit of DiMA's radio signal and her not understanding why she cannot accompany her father on trips out to the wastes. She is frequently bored, as she is isolated from the wasteland in her remote home. Her grandfather's passing worsened this feeling of boredom.
Kasumi is in fact human. If the Sole Survivor chooses to destroy Acadia, synth components are lootable from all the synth corpses, but not from Kasumi's.
Reporting Acadia to the Institute is an optional quest if the Sole Survivor is on friendly terms with the Institute. During the conversation with Dr. William Moseley in Advanced Systems, you can inquire about Kasumi and Moseley will reply that they have no record of any synth deployment with that identity. He raises the possibility that her personality may have been given after a memory wipe but offers no other confirmation.
If the Sole Survivor chooses to report Acadia to the Railroad during an optional quest, the Railroad operative reports that she doesn't recognize Kasumi's name and that they never helped a synth with that identity.
Her first name, Kasumi (霞), means "haze" or "mist", and her last name, Nakano (中野), means "in the field" in Japanese.