in short , reign is the son of the attack titan preceding both eren & grisha yeager ; eren kruger , or , as he is better known , THE OWL . reign was the accidental ( but adored ) result of a sporadic affair between kruger & an eldian woman posing as marleyan . kruger , still hiding under the guise of a soldier , spent a mere two years with his son before allowing himself to be consumed by grisha to pass on the titan .
reign & his mother knew virtually nothing regarding kruger's sudden disappearance , allowing themselves to mourn briefly before moving on with their lives . however , the peace did not last ; it was only a week later that kruger's affair was discovered , his lover killed , & his son taken by the military . initially , they planned to kill reign — only for the argument that , ' if anyone could manage to steal back the attack titan , it would be the son of one of the inheritors , wouldn't it ? '
thus , reign was introduced into the warrior program at a VERY young age . his heritage & purpose within the unit was finally explained at the age of eight — & lest to say , he loathes both his father , & the yeagers for handing off the legacy that should have been his .