I don't know Japanese, so I can't tell if soulmate is just a weird translation (or a weird translation from a weird translation as may be the case
)or not. However, I have read Anne of Green Gables and the term Anne uses is "bosom friend." It is easy to understand why people who speak english would be confused with the soulmate translation because that term is generally used in reference to a romantic relationship.
"Bosom friend" puts a more platonic spin on the relationship. I would think they are more like best friends.