So here's what I think... at the end where the seme died, in order for his mission to be c...

So here's what I think... at the end where the seme died, in order for his mission to be cleared and him not disappearing into thin air he did take the seme's soul as promised but in order to save the seme's life, he sacrificed his own soul and give it to the seme. Thus now the seme has the reaper's soul instead therefore I think this is why the seme is able to remember the reaper because it had the reaper's memories and all which is also why he didn't freak out when a random guardian angel appeared.
For the reaper the reason why he cannot remember shit may highly be because he gave his soul to the seme which has his memory and all. And because the reaper did not fail his mission he was not disappeared to think air and also because he is not human he cannot die so I think he was just given a new soul instead. And why he became a guardian angel instead may be because he became pure and knows nothing about being a reaper.
This is just my theory