Frankenstein

After the atomic bomb which destroyed Hiroshima irradiated his immortal heart, it slowly regrew into a feral boy, and eventually a giant. Frankenstein became an outcast, blamed for the crimes of the underground monster Baragon and hunted by the JSDF. Ultimately, he confronted and killed the monster before a sudden fissure sent him plummeting into the Earth's depths, his fate unknown. Though referenced in The War of the Gargantuas as the ancestor of Sanda, Toho's Frankenstein has not appeared in any media since then.