The Manchester United defender is on a six month deal without a permanent option.
“Axel is very excited to begin a new chapter in his football career, playing abroad and for a club with so much history behind it and fans who worship football," Dimitri Tuanzebe of Millennial Sports Management told ESPN.
“Axel didn't decide to move until it became very clear that he isn't the manager's first-choice centre-back and the Napoli offer was very tempting.
“For the avoidance of doubts, Axel was very open to staying at Villa, he loves the fans, being in Birmingham but he also has a career that he can only build and reach his full potential in an environment where opportunities to play are fair."
Tuanzebe has a contract with Manchester United until June 2023, although the club does have an option to extend for another year.
“Man United was very supportive of the move, made sure that it was in favour of Axel's career and Napoli did everything within their power to make the deal possible. As Axel's agent, I made sure that the deal went ahead by working in collaboration with trusted partners in the UK and Italy.
“Man United contacted me directly to notify me of Napoli's interest, I added them to the list of clubs interested and then assessed all options with the help of my family, trusted partners and Axel.
“After multiple meetings with interested clubs' sporting directors and managers, we all concluded that Napoli is the best option.
“Axel was persuaded by the clear path into the team, also a success story of players who have enjoyed playing time since they signed for Napoli after a lack of opportunities at their previous clubs."
“Axel will play his part when called upon by the manager. Barcelona is one of Europe's top clubs, filled with an attacking force. Axel just wants to play football and show everyone what he can do, especially against one of Europe's giants."