I think that Jaeyi knows that Ideum is with his brother, cause it was Jaeyi who recommend Ideum to his friend which is an instructor. So Jaeyi probably called that friend but that friend told him they are not having lunch, and probably told me that Ideum is with his brother
I think that Jaeyi knows that Ideum is with his brother, cause it was Jaeyi who recommend Ideum to his friend which is an instructor. So Jaeyi probably called that friend but that friend told him they are not having lunch, and probably told me that Ideum is with his brother