After restoring a radio and contacting DiMA, she is led to believe that she is a synth; this is reinforced by her inexplicable dreams about waking up in a lab and innate mechanical ability, although she does have her own doubts as to if this is the truth, nor can she prove it.
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 empathetic, as shown by her pursuit of DiMA's radio signal and her desire to help the people of Acadia. When she was living with her parents, she was frequently bored, as she was isolated from the wasteland in her remote home. Her grandfather's passing worsened this feeling of boredom.
There is no evidence to support Kasumi's belief she is a synth, beyond her own feelings. Going to the Railroad or the Institute can lead the player character to have conversations with Boxer or Dr. William Moseley, respectively, about whether Kasumi is human or synth, but neither conversation confirms or denies Kasumi's belief.
Kasumi does indeed appear to be human. If she is killed, she will not drop a synth component.
Even if Kasumi is "reclaimed" with the synths of Acadia for the Institute by informing Dr. Moseley about Arcadia for Forbidden Knowledge, the player character is never told whether she was human or synth.
Should Acadia (and Kasumi) be reported to the Institute, Dr. William Moseley will indicate that they have no record of any synth deployment with that identity, meaning she cannot be a synth under Institute control. He does, however, raise the possibility her personality may have been given after a memory wipe.
If the Sole Survivor chooses to report Acadia (and Kasumi) to the Railroad, the Railroad operative reports that she doesn't recognize Kasumi's name and that they never helped a synth with that identity, indicating that she cannot be a Railroad assisted synth.
↑ 1.01.1Kasumi's final holotape - It sounds wonderful, but... then they started asking about me. And some questions came up. Questions I don't have answers to. I mean... I've always felt... off... like I'm not really supposed to be here, but then there are things in my childhood that I can't remember, and I've been having strange dreams...
↑Kasumi's dialogue - Line 104: Little things. Dreams of waking up in a lab. Years I can't remember from when I was a kid. Not to mention how I never quite fit in back at home...
↑Kasumi's dialogue - Line 93: Yeah, so am I... I mean... yes, I believe I'm a synth, but... there's something wrong here. With Acadia. There's more going on than just the refuge.
↑Kasumi's dialogue - Line 202: I... Yes it's true... I mean... I can't prove it, but when I made it to Acadia, I just knew...