Ningirsu is the oldest of the three, him the older brother of Ib. His text lore reads this (Using the OCG [Japanese version] as it gives more information):

A young man who spends all his time fighting Mekkstrosities.
Even when the Krawlers living around the forest suddenly went on a rampage, he stood his ground and prevented them from breaching the barrier.
His valiant form standing at the front lines is a source of inspiration for the denizens of the forest, but he wields his spear for the sake of his sister.

From the look of his card art, he seems some-what calm and composed at most times, being the adult figure I suppose. I personally like to think of his voice as a slightly higher pitched Katakuri or All-Might (Really a higher pitched voice than any anime character with a deep voice).