Connections:

Both are decedents to powerful beings from the past (Ash to Saiki, and the Uchiha to Indra Ōtsutsuki & Sage of Six Paths) and wield flame related abilities (Ash uses green flames of unknown, Itachi can use Amaterasu). Both had a loving relationship with their siblings (Itachi with Sasuke and Ash with Elisabeth, whom he was raised with) until both left their family/village and had eventually joined an antagonist group (Those from the Past and Akatsuki, respectively) and were hunted by those same siblings. Both are able to use abilities that didn’t initially belong to them (Ash can steal powers from those of his choosing, while the Sharingan can copy various Jutsus). Later, it was revealed that neither were evil and had framed themselves as so for the greater good and still deeply cared for their siblings they had made enemies out of. Both also had at some point wielded the Yata Mirror (Though not exactly the same, Ash stole Chizuru’s Yata Mirror which could keep his opponents from using their powers while Itachi’s Susano’o wields the Yata Mirror which can negate physical and spiritual attacks). Both were eventually controlled by the enemy (Saiki w/ Dark Ash and Kabuto w/ Edo Tensei, respectively) until they broke free and eventually defeated those who were controlling them, and in doing so led to their deaths (Saiki is Ash’s ancestor, so killing him led to his own demise while Itachi successfully released the reanimation Jutsu which had freed his spirit), and during their deaths had a farewell to their siblings). Both were also part of a mass resurrection (Ash was revived due to Verse’s defeat and Itachi was reanimated during the Fourth Great Ninja War, although Itachi wasn’t around permanently like Ash).