Brunner played for the academies of Rot-Weiss Essen and VfL Bochum, before joining Borussia Dortmund in 2020.

[5] These goal-scoring feats earned him comparisons to teammate Youssoufa Moukoko, who was similarly prolific at youth level.

[10] Brunner scored his first senior goal for Cercle Brugge on 19 December, matchday 6 of the UEFA Conference League, in a 1–1 draw against Başakşehir.

The late equaliser helped his team qualify directly for the round of 16 by finishing eighth in the league phase.

[1] 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.