He was an erratic genius, brilliant, irritable, exacting, tireless, all but impossible to maintain any consistent relation with but one of bitter enmity.

incansable, indefatigable, tireless, unwearied; adv., tirelessly.

Quick as a cat, supple, powerful, tireless, he kept on the go, whirling, bounding, leaping, rolling, till it seemed we would never catch him.