Super Paper Mario Theory:
Theory: Grambi has not always been the ruler of the Overthere.
By: Drake Inferno and General Donitsky (Graphic by General Donitsky)
Argument 1: Though Nimbis are immortal, they do age, albeit slowly. Luvbi, despite having been born “soon” after Jaydes and Grambi met (and thus being around 1500 years old) is still considered to be a child, and some Nimbis are said to be “young” or “grown up”.
Argument 2: Jaydes and Grambi are said to have met through work shortly before having Luvbi. The magic that controls life and death is called "Jaydes Magic", implying she has ruled the Underwhere since the dawn of life. If so, it would seem strange for the two to have never met until 1500 years ago if Grambi had ruled the Overthere for eons prior.
Argument 3: Rebbi, Yebbi, and Blubi, powerful Nimbis in governmental roles within the Overthere, all look significantly younger than Whibbi and Grambi, implying that leadership roles are not always held by the same Nimbis forever.
Argument 4: Hagnes tells the story of how she tried to gain Grambi's affection in her youth, and how ultimately Grambi fell in love with Jaydes instead. Her phrasing, emphasis of her own youth, and use of "fellow" appears to imply that he was not vastly older than her.
Argument 5: The mural of Grambi and Jaydes in the Flopside Pit of 100 Trials shows Grambi with a more brightly colored beard and eyebrows. Though this is a stylized depiction, other murals (like those of Croacus I or Merlee’s ancestor) account for the passage of time since their creation. Thus, the blue tint of Grambi’s hair in the mural could be meant to depict his appearance before it turned white.