Sephiroth tilts his head. "How are you kept alive without a heartbeat?" he asks, genuinely curious. "Surely you still need something to circulate blood...even an artificial heart would need to serve the same function as a real one."

He turns his attention back to the cat, which seems to be very demanding of attention now. "Before I was born...I was injected with the cells of an ancient, sentient planet-killing virus. It had a dramatic effect on my development, to say the least. How much of me is actually still human is...questionable."

Yeah, Sephiroth, like Byleth knows what cells are. Though, oddly, prior to this, Sephiroth hasn't encountered much in the way of technological or scientific knowledge mismatch in this place...yet.