Generally, 信者 (Shinja) means “a believer or devotee of a religion”, as in “キリスト教信者 (a believer in Christianity)”. However, it is also used as a slang word that means “a fanatic fan” with a negative connotation. For example, “Apple shinja” indicates fanatic fans of Apple, who always form an endless queue yearning for new Apple products. Similarly, you can say “Abe Shinja” to describe fanatic supporters of previous Japanese Prime Minister, Shinzo Abe.
Antonym: アンチ (Anchi)
アンチ (anchi) is an antonym of shinja and means “hater“. It is a loanword from “anti” in English, and indicates someone who intensely hates something. For example, “anchi Abe” or “Abe anchi” describes people who are very critical of Shinzo Abe. Famous and rich people usually have both shinja (fanatic fans) and anchi (haters).
信者 (Shinja) consists of two kanji characters “信” (shin) meaning “believe” and “者” (ja) meaning “person”.
That Youtuber has a lot of both shinja (fanatic fans) and anchi (haters).
According to fanatic fans of ONE PIECE, you can understand the “meaning” of life by reading ONE PIECE.
(note) “ONE PIECE” is one of the most popular Japanese mangas.
Synonyms: ファン (fan)
Synonyms in English: believer, a fanatic fan, zealot
Related words and phrases: アンチ (anti, hater)