Watching anime during Ramadan can be permissible if the content is free from any religious prohibitions such as promoting disbelief, immorality, or magic. Fasting persons and other Muslims should avoid watching indecent content that includes forbidden things like images of naked or semi-naked people, evil words, or actions against Islamic laws. Simply watching such content does not reach the level of disbelief (kufr), as long as the viewer does not accept or is pleased with the disbelief it contains.

And Allah knows best.

Sources:

1. Watching Movies in Ramadan
2. Can you watch TV during Ramadan?
3. Watching animated movies and shows that contain matters contrary to Islamic belief or associate others with Allah