Not much is known about Rainer besides what info was given on the Petscop channel in the description.
The channel description is as follows:
"Rainer" gave this gift to us on Christmas 1997 and 2000. It was the single longest day of our lives. We were all certain he was dead at the time. He had been missing since June 1997 and 2000.
We're not as concerned about these things now.
Please enjoy the recordings! We do. :)
There is also the statement in Petscop 9 about the rebirth of a friend as their daughter, and at the end it is signed "-Rainer, Newmaker."