The unknown student that Yuri learned on her quest to defeat the Shadows in Episode 12. The AI that she encountered tells her of a human who, long ago, turned himself into an NPC to await the miniscule chance that the girl he fell in love with would reappear in the Afterlife. He is referred to as The Programmer as he was the one who made the NPC-to-Shadow conversion program and the maker of The AI which Yuri talked to.
The Programmer's real name is unknown. But like Otonashi, he was never supposed to have entered the Afterlife, but somehow ended up there anyway. He may or may not be the maker of the Angel Player program, as it existed before Kanade ended up in the Afterlife. 
During his time in the Afterlife School, he fell in love with an unnamed girl who disappeared when her regret was fulfilled. Unwilling or unable to accept that, he decided to wait for her to return, which from the Afterlife School rules would require her to:
- Die during her teenage years (13 to 17 or so for a 4 year gap)
- Have a heavy regret
If the girl lived an average of 85 years and reincarnated each time, there would be a 4/85 (teenage years)*9.83/1000(Japan death rate )*1%(Ad hoc chance of heavy regret)=~0.0004% every reincarnation. Each failure to hit this chance would lead to 85 years of waiting. A 10% chance to see her again would require 2.125 million years. The above calculations do not consider that the majority of the death rate is actually from the elderly rather than from teenagers.
As expected, the amount stunned Yuri, due to the massive time required to be spent waiting. The AI explained that The Programmer lost his grip on sanity after waiting so long, and decided to create a program to turn himself into an NPC.
As The Programmer felt that people who had resolved their sadness should not clutter the Afterlife School (much like patients should be discharged when they are cured rather than staying in the hospital), he created a program that kept track of the amount of love in the Afterlife School world. When the amount of love exceeded a certain amount, a "cleaning" program would be activated where NPCs would be changed into Shadows, which would in turn target humans and forcibly change them into NPCs, thus resetting the world into one where no humans existed.
He still exists as an NPC in the Afterlife, but who he is or what he looks like is not known. It is not known whether The AI that Yuri talks to is based on The Programmer's real look.
The AI that Yuri talks to states that she wouldn't know The Programmer even if he did say his name. He also stated that he did not create the Angel Player program when Yuri asks if he knew about it as he replied, "Beats me. It's not like I invented it".
The Programmer must have been a very intelligent student to be able create such a program like the NPC interface in the Afterlife; such advanced technology that doesn't even exist in the real world.