Everything to Know About Ace’s Fate in One Piece: Episodes, Canon, and Legacy
Ace possessed the Mera Mera no Mi (Flame-Flame Fruit), a Logia-type Devil Fruit granting complete control, generation, and transformation into fire. Up until Marineford, Ace appeared invulnerable to standard physical harm. Akainu wielded the Magu Magu no Mi (Mag-Mag Fruit), another Logia ability based on molten rock.
Eiichiro Oda established a strict environmental hierarchy between the two fruits. Magma consumes and smothers fire by virtue of mass, density, and sheer elemental heat. As Akainu explicitly stated before the fatal blow, his magma possessed the power to scorch fire itself. Ace could not phase into intangible flames to let the strike pass through him; doing so would have allowed Akainu's magma fist to obliterate Luffy standing directly behind him. Ace intentionally solidified his flesh, absorbing the strike entirely with his physical body to act as an absolute shield for his younger brother.