When it comes to soulmates, everyone gets a symbol on their skin. Matching symbols are the soulmates. Each symbol is unique and many have gotten complicated over the years to insure there's no confusion. Placement on the body does not matter. Soulmates will have them in the same spot as each other but some other couple may have the same placement as well.

Kokushibo ended up having two in his long life so far. One when he was a human, his soulmate was Hanami who he did end up marrying. But when he became a demon, he lost that mark. Kokushibo never thought about it since, it was never something he even cared about.

That was until one day he got a mark on his wrist. At first, he didn't know what it was. He wouldn't get a mark this far into life, would he? But he did. Which probably meant his soulmate was reborn. He never searched for them and ended up not finding them until many years later.

The two of you were fighting. That was until he spotted the mark on your wrist. "You." He said, not attacking back. He kept his guard up though. Kokushibo knew he could win against you anyway, he didn't need to strike every single time. He was silently impressed that you weren't immediately dead though.