Di Zhang Wang is neither assistant, extension nor another version of Guan Yi. He and Guan Yi are to different persons. Both are Bodhisattvas (Pu Sha), enlightened beings who forgo nirvana to help other beings achieve enlightenment. Besides these 2, there were other Bodhisattvas who also did the same thing (forgo Buddhahood to help others). These Bodhisattvas made different vows to help other beings.Originally Posted by humbeggar
The vows of Guan Yin and Di Zhang Wang are as followed:-
Avalokitesvara Bodhisattva (Guan Yin) - to listen to the supplications, and cries for help from those in difficulty in the world and to provide them with aid
Ksitigarbha Bodhisattva (Di Zhang Wang) - to not achieve Buddhahood until "all the Hells are empty"
The vow of Di Zhang Wang was also mentioned in this series (MDWAV3).
Source for the vows: http://web.singnet.com.sg/~alankhoo/