Touhou Shinigami: meteoras Methuselah Gaidenas

A miniseries prequel about the encounter between Rain and Zol.
This is a prequel to Immortal Rain set before Machika’s birth. Machika’s grandfather Zol, who works as a bounty hunter by the name of Grim Reaper, encounters Rain, who turns out to be Methuselah, the most wanted man in the world.
Why did fate bring these two men together and what is hidden behind Rain’s clumsy and kind personality?