Well, the idea of 'Casper the Friendly Ghost' has some roots in what could be considered a 'true story' in the sense of inspiration. It was developed from a concept that its creators had, perhaps from a general idea about a ghost that was different - friendly instead of scary. So, while not a traditional 'true story' like a historical event, it has a origin story that was inspired by something real in the minds of its creators.
There is a basis in truth to the 'Casper the Friendly Ghost' story. Seymour Reit and Joe Oriolo, the creators, took an idea that was floating around, maybe from a folktale or just a creative thought, and developed it into Casper. Casper defies the typical image of a ghost as something to be feared. His friendly nature was a unique take, and this all started from that initial inspiration which can be seen as a form of a true - story - like origin.
Yes, it is based on a true story. Casper was created by Seymour Reit and Joe Oriolo in the 1930s. They were inspired by a story they heard about a friendly ghost, and thus Casper was born. This friendly ghost has since become a very popular character in various forms of media, from cartoons to movies.