*Club domestic league appearances and goals, correct as of 13:42, 14 November 2022 (UTC) ‡ National team caps and goals, correct as of 04:54, 26 March 2024 (UTC)
He rose to prominence in the footballing world after scoring sixteen goals in only five Under 17 Bundesliga appearances.[4][5][6] He was promoted to the under-19 side at the age of sixteen, scoring on his debut against Bonner SC in the Under 19 Bundesliga.[5] These goal-scoring feats earned him comparisons to teammate Youssoufa Moukoko, who was similarly prolific at youth level.[5][7]
On 11 October 2023, he was named by English newspaper The Guardian as one of the best players born in 2006 worldwide.[8] A week later, Borussia Dortmund indefinitely suspended Brunner for undisclosed disciplinary reasons.[9]
During the 2023 UEFA European Under-17 Championship, he scored the equalizer for 10-man Germany team against Switzerland in the quarter-final match which ended in a 3–2 win on penalties, securing a place in the FIFA U-17 World Cup.[12] He later managed to win the competition with his country after beating France in the final 4–3 on penalties. He was also voted player of the tournament, in addition to being the joint top scorer with four goals.[13]