Is there any connection between Otonashi and the first NPC? In episode 12, the Mysterious Boy (who is probably the first NPC) explained to Yuri that there was a boy from a long time ago who loved another girl who left him alone in this world. Thus he lost his grip on sanity, and created some sort of "requirements" to this world (the main one being to not allow love to spread). But the Mysterious Boy also told her that his chances of meeting her again are better than 0.

Based on this question, it is assumed that Otonashi didn't have regrets so he did passed on. But I beg to differ on that, because we can also assume Otonashi DID have regrets right after Kanade left him even when he begged for her to stay with him.

It's all based on assumption really, but could it be that the first NPC was actually Otonashi himself in some way?
Theoretically, no. It's true that Kanade arrived before Otonashi and Otonashi died in the real world before Kanade (because Otonashi's heart went to Kanade), but I think this question never received a response from Key.