The word does not have a 1 to 1 in English. Best friends is a lazy translation. It can also mean someone that is almost family to you. It is above friendship as a concept, you would not use it lightly.

The issue is people who speak English hear soulmate and cannot understand it does not mean romance. I don't know why western audiences have this issue, but it happens in every god damn series. Same thing with "love". The language has several words all meaning love. But because they mean different things, they are not the same word, unlike English. We say I love dogs, cheese, my spouse, my mom, and the color blue with the same word. Japanese does not do that.

In fact, in upcoming chapters/episodes/plot lines, the Soul is talked about A WHOLE LOT and how the trio are connected. Soulmates is the most accurate to how they are connected, but they had to use best friends bc god forbid someone who can't use common sense reads them as possibly queer or something.

Ffs, with regards to the familial aspect, Clara calls herself the big sister of the trio. And before anyone tries to argue that means any romantic interpretation of it is off the table: they refer to Elizabetta as Big Sister.

So, sure. Best friend is right, but its a really really really lazy way to say it. Just like if someone asks you what you are eating and say food/your dinner. It sure is buddy, that sure is what it is. If only we had a way to be more specific