The parasite was the result of the Demonic Form, one of Noah's oldest spells. The creature didn't feel the same affection as the other companions, but it held a special place in his mind due to its connection with the technique that had allowed him to reach his current state.
The transformation into a proper living being had enhanced its destructiveness, but it had added troublesome features to its existence. The parasite fed on Noah's energy and tissues, but the black hole limited that process. That slowed down the plant's growth and allowed him to control when to give it enough power to trigger the breakthrough.
Noah wanted his body to be near the ninth rank before triggering the plant's breakthrough. He feared what the parasite would be able to do after stepping into the ninth rank, but Dinia didn't give him other options.