For revenge.
The third person that Celebolo parasitized was Yaohui. This was probably something he had planned for a long time, both for revenge and for fun. This time, Cerebro made a lot of money from parasitism and directly obtained Zeta's body. However, the operation is not very smooth because Yaohui's will is too strong.
Celebulo, the monster that appeared in the Japanese special edition "Ultraman Zeta", first appeared in the first episode. The name comes from the Spanish word cerebro.