Board Thread:Chapter Discourse/@comment-36009152-20190603231808/@comment-36009152-20190604124622

It is speculation, it could be anything: though reincarnation is possible it raises questions since the two Yu’s are just so different. Again guesswork, but rather than reincarnation, genetic engineering or concepts we are familiar with, it may be something different that we are not familiar with for Seraph of the End. Something along the lines of an essence transfer, to more likely a blood transfusion that would transfer the powers of the Yu from the past into the present Yu.

That his parents then found out about when Yu had grown a bit, that may explain their reaction, but again these double Yu’s are a large mystery of the plot. Complicated further here since this past Yu is definitely a seraph, and one Sika Madu is familiar with and how to manipulate on various levels.