This is an explanation from the novel:
All of Jaeui's gray matter and energy are used to sustain his once-in-a-millennium genius brain, so his internal organs don't have enough energy to sustain themselves (cells become aged, blood is depleted, etc.). This is the fair trade-off for Jaeui's genius mind.
- Taeui - the appearance of Taeui is a non-physical factor. Taeui has the lucky ability to sustain Jaeui's life. If Taeui dies, Jaeui will die too, but if Jaeui dies, Taeui will still be fine.
- However, no one can touch Jaeui. With Taeui's luck and Jaeui's genius brain, Jaeui can be eternally immortal and destroy the world.]

I am so confused what is even going on atp