Did Kaneki Really Say “Never Trust Anyone Too Much, Remember the Devil Was Once an Angel”?
Unverified. No chapter, episode or licensed English text we could find gives Kaneki this line; it circulates on quote sites with his name and nothing else.
Unverified. "Never trust anyone too much, remember the devil was once an angel" is shared as a Ken Kaneki line from Tokyo Ghoul. We found no source that ties it to the series: no chapter, no episode, no licensed English text and no editorial outlet. The quote sites that carry it give only the name; one we checked, MyAnimeThoughts, lists it as "Kaneki Ken · Tokyo Ghoul" with no location. Not finding a line does not prove it was invented, so the verdict stays open.
What we checked
Wikiquote has no Tokyo Ghoul page, and a full-text search of the site for the sentence returns nothing. Searches for the line together with Kaneki surfaced user-voted lists and quote aggregators, which this site does not accept as evidence, and no article from an outlet we trust. We did not read VIZ's English volumes or watch the English tracks of the anime, so a line we have not seen could still exist.
Where the idea comes from
The second half restates an old Christian teaching. The Billy Graham Evangelistic Association puts it this way: the Bible "does indicate that he was an angel who rebelled against God and was cast out of Heaven as a result." That tells us the thought is older than the series. It does not tell us who first joined it to Kaneki's name.
What would settle it
A page of VIZ's English Tokyo Ghoul or Tokyo Ghoul:re, or a named anime track, that prints the sentence. An early post that pairs the sentence with Kaneki's name and gives a chapter or episode would at least show where the attribution started. The series is in the Tokyo Ghoul buying guide; the verdict is logged in the fact-check database, and the verification method explains why an unfound line is unverified rather than fake.