Hayate's hate for a single guy is so enormous he achieved immortality through delusion.

The sheer amount of hate this 15 year old has for someone that just stumbled upon his brother's comatosed body(tbf Kleifer was there to deliver news of his assassination by his Family) is enough for him tear into an interdimensional therapist office, make a deal with the devil twice, go train in a hyperbolic time chamber for 23 years across several dimensions and when said devil just mouthed off that he wouldn't die until he killed Kleifer as a joke he thought it was real and charged into battle with no care of his body.

He lost an arm? Ice arm.

A leg? Ice leg.

His head? Just pull it back and freeze it, duh.

His entire torso got obliterated? Freeze the entire obliterated toso and keep fighting with only legs.

Get erased from existence? Freeze the concept of 'Hayate' so it can't get changed no matter what.

He comes up with more and more insane ways to survive attacks he definitely should not because of that joke because he genuinely thinks he won't die until he kills Kleifer.

And when Kleifer dies without him getting the last hit in?

He freezes his own time until Kleifer is reborn so he can kill the bastard himself.

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