Kagerou Kalan!
Kagerou is a strange case. While he is a firebender, he also can't technically firebend. When he was born, Kagerou nearly died of a birth defect that affected his lungs, making it more difficult for him to breathe properly. Now as a teenager, Kagerou is still affected by this defect when he does heavy physical activity, but he's learned to live with it over the years. However, his body and his lungs can't handle the strain firebending puts on them so even though he was born a firebender, he'll never be able to do it effectively without putting his body at risk.