Why do i feel like that "twin" brother is just mcs imagination

reminds me of that webtoon my deepest secret

If you mean he never existed in the first place, that wouldn't make any sense since in the flashbacks we see people being aware of them being twins

No like maybe he actually died in the fire

Ahh I see, that's more possible but I dont see why yui would hallucinate his brother going to jail and all when at some point he returns anyway