I agree that I need to rewrite and add on to that particular paragraph. Basically, what I meant was that if someone was self-inserting them into a fictional universe (that doesn't resemble the real life universe) in the role of having been born there rather than being dropped there through a portal or something. If someone was supposed to be born in, for example, Naruto, the name Cassandra wouldn't be universally acceptable.
If the writer absolutely wants to keep their name, I say they can convert their name into katakana, or they can find the meaning of their name, and choose a Japanese name that means something similar.