I came to the headcanon that Grubber was kind of a runt, seen as useless and passed around through the Storm King's forces because of his size and aversion to combat, until he landed in Tempest's forces, where she sort of saw him as a kindred spirit of sorts, outcasts among their kind, so she kept him around.

After the fall of the Storm King, I like to think they'd eventually develop a similar relationship to Twilight and Spike. Not just comrades in arms or friends, but family.