When Ach appeared, it caused the Rainbow Whale to have an extreme reaction and turn around to flee.
However, after Liszt's soothing and half an hour's struggle, it finally accepted Ach's presence and was no longer afraid of the Sea Serpent that had captured it before.
It was all thanks to Liszt letting Ach catch some small fish and throw them to the Rainbow Whale that he successfully eliminated its hostility towards Ach.
Food is always the best tool for communicating with animals.
But what moved Liszt the most was that, while avoiding Ach, the Rainbow Whale never once threw him off its back, nor did it cancel the protective magical layer covering him. Carrying Liszt while dodging Ach greatly reduced its speed; otherwise, with its velocity, it could have easily gotten rid of Ach.
Ach was a sprite of the sea, much faster than ordinary fish, but still far behind the Rainbow Whale.