Complex sentence: Itachi's character had been popular within readers from the manga, and was positively received by critics; his appearance as an antagonist has been praised by several writers, although some considered his initial to be unsurprising.
Split into simpler sentences: Itachi's character had been popular with readers of the manga, and was positively received by critics. His appearance as an antagonist has been praised by several writers, although some considered his initial appearance to be unsurprising.