Will Shido become a spirit?

Could you elaborate on the possibility of Shido becoming a spirit, as mentioned in the passage? Is this a development that's been hinted at throughout the story, or is it a sudden twist? What factors might contribute to such a transformation, and what implications would it have for the plot and the characters involved? Are there any parallels or similarities to other stories where a protagonist undergoes a similar change? Additionally, how would such a development affect the readers' perception of Shido and the overall narrative?