Agni's regeneration is strange.
I reread Fire Punch a few days ago, and I realized that Agni's blessing being classified as regeneration is a bit strange. (It's worth remembering that in Behemdolg, Jack is considered a regeneration Blessed, even though his ability seems more like accelerated healing, as shown when he healed Sun in Volume 1.)
In Volume 3 or 4, Togata talks about the core of regeneration Blesseds, implying that this core is what grants them their regenerative ability. In the same conversation, she states that Agni's core is located in the right side of his brain.
However, in Volume 3, during Agni's fight against Doma, he is caught off guard and—judging by the somewhat messy artwork and the amount of information packed into a single panel—his skull appears to be crushed. Assuming his regeneration truly depends on a physical core, he should have died or at least lost his regenerative ability. Instead, nothing happened and he simply kept regenerating.
Similar events only appear again in Volume 8 during his fight against Sun. After being reset by Judah and later struck by the meteor impact, Agni was hit by the electrical beams that were destroying the houses and the village itself. Immediately afterward, his skull was pierced by Sun's staff, and he was electrocuted internally. If that wasn't enough to destroy Agni's brain, I don't know what would be. Yet his regeneration still didn't stop.
When Judah reset him, the right side of his brain was destroyed, and he only reappeared a few panels later. But the most problematic event was the collision between the meteor and Earth.
Let's look at the numbers.
To completely destroy Earth to the point where it would fragment into multiple pieces incapable of reassembling under their own gravity, an impact releasing approximately 3 × 10³² to 5 × 10³² joules of energy would be required. This exceeds Earth's gravitational binding energy, which is roughly 2.24 × 10³² joules.
An impact of that magnitude would require a body with a mass comparable to a planet—likely a rocky object between 8,000 and 12,000 kilometers in diameter traveling at 20–30 km/s. This would essentially be a planetary collision rather than a conventional asteroid impact.
At the moment of impact, pressures would reach around 10¹²–10¹³ pascals, while temperatures at the impact site would climb to tens or even hundreds of millions of kelvin, exceeding even the temperature of Sun's core. Within the first milliseconds, the atmosphere would be compressed and heated to the point that it would cease to exist as a gaseous mixture, becoming fully ionized plasma. Shock waves and seismic waves traveling at hundreds of kilometers per second would then propagate across the entire planet, while Earth's surface would begin to be torn apart and accelerated to orbital velocities or higher.
The energy required to completely evaporate Earth's oceans is on the order of 10²⁷ joules, less than one hundred-thousandth of the total impact energy. As a result, all surface water would vaporize almost instantly. Earth's crust would melt and partially vaporize, enormous volumes of the mantle would be ejected into space, and even material from the core would become exposed and dispersed. Within minutes, the distinction between atmosphere, oceans, crust, and planetary interior would vanish entirely, replaced by an expanding mixture of plasma, silicate vapor, molten iron, and debris.
Cities would leave no recognizable ruins. Concrete, glass, steel, and every other construction material would first be heated, then melted, vaporized, and in many cases ionized. Likewise, every living organism would be destroyed without exception. Near the impact site, bodies would be converted directly into plasma by temperatures of millions of degrees before the shock wave even arrived. Even on the opposite side of the planet, the combination of extreme thermal radiation, surface ejection, and immense pressures would result in the complete vaporization of biological tissue. No surface, underground, or marine organisms—including extremophiles adapted to the harshest environments on Earth—would survive.
This leaves three possible explanations:
Fujimoto simply forgot about the core.
Agni's blessing is not actually regeneration.
Agni's core is non-physical.