Needless to say, if Ojii ever met another Ren...
They'd be hugging and crying it out.
In his dimension, Ojii traded his mortality for Ren's immortality in order to let him come home and live a better life, but it was short-lived... Soon after the apocolypse came, his Ren died due to starvation.
Ojii still can't get over it, and he's stuck in a confused state about if it's his fault for the choices he had made.
Enjoy!
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(RP, anyone?)