In his youth, Peter was a slender build man with black hair. Over the years, he got a bit bulkier, and his hair turned white. He also grew a beard.
In the animated series, his personality is not very clear, but he is described as kind and courteous, however, because he appears in few episodes, he has not been described very generally.
Peter has been living in Trolberg for at least the past 50 years.
50 years before the events of the series, Peter went to the Mirage nightclub, and met a young Matilda Pilqvist there. They spend a wonderful night together, and Peter fell in love with her, but he didn't want to seem too eager and thus the two parted ways that evening without even learning each others names. Peter hoped he would see her again, which never happened. Over the years, he came to consider not making a move when he had the chance his biggest mistake, and tried to relive that night in any way he could. Unbeknown to him, Matilda had also fallen in love with him, and likewise regretted how things had turned out. In a moment of recklessness, she enchanted the copy of the Trolberg Digest magazine that Peter had been reading that night, granting it and every copy of it the power to let the reader travel back in time to the night they first met.
At some point before the series, Peter bought a copy of the magazine and thus discovered it's time travel abilities. While he didn't understand how the time travel worked, he gladly used it to revisit the night he met Tildy and watch it all happen again from a distance. He began to order more copies of the magazine, but had to be careful not to overuse them, since every trip to the past resulted in another version of him ending up there.
In "Chapter 8: The Fifty Year Night", Hilda discovers Peters secret when she watches him time travel. She gets her hands on a copy of the magazine and follows him into the past. After learning about Peter's story, Hilda decides to use the magazine to fix Peters mistake and get him and Tildy together. Her plan works, resulting in an alternate timeline in which Peter and Tildy are happily married, but it also alerts a Time Worm that begins to devour the old versions of Peter, Hilda and Tildy, since they should no longer exist in this new timeline. Tildy is able to hold the time worm at bay long enough to inform Hilda that she has come to regret her actions, and that the past is best left unaltered. The alternate Tildy and Peter agree, and destroy the original magazine, thus breaking the spell and undoing all the changes caused by the time travel. When Tildy brings Hilda home, she finally meets Peter again in the present day. Peter no longer remembers his time travel adventures now that the original magazine is destroyed, and Tildy decides not to tell him who she actually is.
The events of "Chapter 8: The Fifty Year Night" are also adapted into the book "Hilda and the Time Worm", with a few notable differences. While the animated series gives no specific date, the book mentions that the night Peter and Matilda spend together was in December 1947, and that Peter was 23 years old at the time. In the new timeline that Hilda ends up creating, Peter and Matilda have been married for 40 years in the present day.
After Hilda's actions in the past attract the Time Worm, one of the 8 copies of Mr. Ostenfeld manages to escape the beast, return to the present, and warns his counterpart from the new timeline what is going on. After the Peter and Matilda from the new timeline sacrifice themselves to restore history and save Hilda, Mr. Ostenfeld retains his memories of the adventures, and reveals he now also owns a picture book now with snapshots of his and Tildy's life togetherk in the alternate timeline. He decides to stop time traveling, and use this book to remember Tildy.
He is briefly mentioned in "Hilda and the Ghost Ship" when Hilda needs to distract her mom, so she lies that she heard Mr. Ostenfeld call for help.