Stories vary on the question of whether Keldeo has become a sword of justice, and many still describe it as nothing more than an enthusiastic admirer. Without question, Keldeo is far younger than Terrakion, Virizion, and Cobalion, and was absent from many of the events which formed the other three's strong bond – and yet even those who still consider Keldeo a demigod apprentice acknowledge that it has been given authority over a fourth sphere; water.
Unova's coastline and rivers often do not fall clearly into the division of forest, city, and mountain, and although collaborative efforts between the three (and the self-interest of much of humanity) have proved successful in stemming the worst incidents of pollution, many minor injustices and disturbances had gone unchallenged until Keldeo was entrusted to the task of protecting them, at which it performed excellently for centuries.
Unova's industrial revolution, however, saw Keldeo's task become far too difficult for its ability to handle, at a time when increased environmental stresses were making it harder than ever for its comrades to help out. The chemicals from many factories polluted the water, and the poison pokemon who came with them displaced the prior water types, which fed a vicious cycle of environmental destruction. Keldeo attempted to raid factories as Virizion had once raided lumber mills, but its efforts were defeated at the site of its attack, for humanity had gained access to many stronger pokemon, and fear of Luddite sabotage saw its targets too well-guarded to defeat.
Yet despite these early failures, Keldeo has perhaps been the most successful of the four in the last few decades. Once reduced to raiding whaling ships and pirates, in recent years it has led swimmers, fishermen, and environmental activists in a massive cleanup of Unova's harbor.