Eagle-eyed fans have spotted what Endrick's younger brother did after being slapped by the Brazilian star during his Real Madrid unveiling.
Endrick, 18, was presented to Real Madrid's supporters in a massive unveiling event at the Bernabeu on Saturday afternoon.
Madrid agreed a deal to sign Endrick from Palmeiras in 2022 in a transfer worth up to €60m (£50m) including add-ons.
But due to only being aged 16 at the time, Endrick was unable to immediately join the European and Spanish champions.
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Instead, he remained with Palmeiras to continue his development in Brazil, playing his final game for the club earlier this summer.
In total, Endrick scored 21 goals in 82 appearances for Palmeiras, while he has already managed three goals in 10 games for the Brazilian national side.
Hopes are high in Madrid that he will go on to become another South American superstar at the Bernabeu, following in the footsteps of compatriots Vinicius Jr and Rodrygo.
Endrick was unveiled by Madrid in front of tens of thousands of supporters on Saturday, with the emotional Brazilian telling those in attendance that the move was a "dream come true".
"This is crazy," Endrick said. "I am so happy, since I was a kid I have always been a Madrid fan and now I am going to play for Madrid.
"I am here, I am going to play, I am so happy, my family are as well. I don't have words to describe what I am feeling. I always wanted to be here. I wanted to play for Madrid. It was a dream. Today it is a reality."
Fans spotted a heart-warming moment between Endrick and his younger brother Noah during the ceremony.
After walking past his sibling, Endrick couldn't resist the opportunity to slap his brother on the head in front of dozens of photographers waiting to take his photo.
Noah turned around in anger before sticking his tongue out in the direction of his older brother.
"Average siblings," commented one user on X.
While another joked: "Bro was like I'm gonna deal with you after this."
Earlier this year, Endrick penned an emotional open letter to his brother in the Players' Tribune, detailing the sacrifices both he and his parents had made to make his life comfortable.
"For three generations, and maybe more, our family has chased the dream of football," he wrote.
"We have been trying to change our circumstances. But now you can do whatever you want. You can be a doctor or lawyer, or maybe since we are going to Spain, the country of [Rafa] Nadal and [Carlos] Alcaraz, you can become a professional tennis player.
"You are already chasing the ball, like me. So you can be a footballer if you want. But you don't have to be. There is no stress anymore, thanks to God, thanks to Mom and Dad, and thanks to football.
"You just enjoy your life how you want, brother. That is my gift to you."
Topics: Real Madrid, Endrick, Football, La Liga, Brazil
Rory O'Callaghan