Boro's midfield man has skippered the side in the absence of Jonathan Woodgate and he is the club's leading goalscorer after taking his tally to six for the season at Huddersfield on Saturday.
The 28-year-old showed nerves of steel to slot home a stoppage time penalty to win the game at Huddersfield, and he will be raring to go again in the big clash with Cardiff.
"He's very important, but not just because of the goals," said Aitor. "I like him because he's a strong character. He always wants to play, and he is good for the changing room.
"He will take responsibility to shoot a penalty to win the game in the final moment, and he's very good for his team-mates.
"We have a lot of players with character and intensity who are a great example for other players."
While Leadbitter is likely to lead the team again on Tuesday evening, the fans could have to wait a while longer before seeing the likes of Jelle Vossen, Patrick Bamford, and Yanic Wildschut in the starting line-up.
"All the new players are feeling every single minute they watch," said the Head Coach. "I have to be calm with the players. Everyone is expecting Vossen is going to play as soon as possible, but he played one game in pre-season and he doesn't know the league.
"Patrick knows the league, but he's a young lad, and Wildschut doesn't know the league. It's my job to pick the right team."
James Husband and Tomas Mejias are both sidelined for the Cardiff game due to injury, along with Connor Ripley, but Kenneth Omeruo could be back in contention after international duty.
Boro were backed by the biggest away following in the country on Saturday with over 3,500 travelling to Huddersfield. That number is likely to diminish to around 500 at Cardiff for one of the longest trips of the season, but whatever the numbers, Aitor says the support deserves to be rewarded.
"It's always been the same since I came here," he added. "They are behind us and helping us and it's very important to have that. Huddersfield was amazing.
"There won't be as many at Cardiff. I know how difficult it is for them with this game. It's a midweek game and people have to work. The best thing is to give them a victory to be happy with. And we want a victory at home on Saturday for those who couldn't travel away."