My Death-Defying Dog: Man's Best Friend, World's Best Savior

The years have felt hollow ever since Hayate lost his beloved dog, Rikimaru. But after losing his ordinary life in a freak accident, Hayate is reincarnated in a fantasy realm whose resident Goddess tasks him with saving her world from impending doom—and gives him the chance to reunite with Rikimaru.

The twist is, Hayate's not the one playing hero. His pup is! Granted the might of a legendary savior, Rikimaru wastes no time in vanquishing evil and enjoying his zoomies. Meanwhile, the good-natured and congenial Hayate is forced to navigate strange, new lands and thorny political intrigue. Still, if there's one thing Hayate knows, it's that he wants to do right by the innocents of this world...and pamper his lovable little pumpkin every step of the way!

(Source: J-Novel Club)