I used to know a conservationist family who rescued an abandoned baby otter and brought it back to health and suitable maturity to survive in the wilds on its own. They even wrote a book about it. As expected, an otter stays more or less completely wild even as a pet. It has zero generational domestication going on, after all, unlike dogs and cats, for example. It's pretty social, though, by nature. Those people were pretty hardcore nature conservation folks, albeit with scientific backgrounds, so they made sure the creature didn't grow up helpless or alienated from nature. So maybe being their pet wouldn't be quite the same as for someone random, who would keep the beast locked indoors, treated like a real pet.
First time back home in five years is a long time. If the missus's only family was a mother who was working abroad for five years, never visiting Japan, it's interesting the family home still remained. Maybe it was rented out to an outsider for that five years predetermined time period.