Enables histone H3-methyl-lysine-9 demethylase activity. Involved in histone H3-K9 demethylation. Acts upstream of or within several processes, including cellular response to ionizing radiation; positive regulation of chromatin binding activity; and positive regulation of double-strand break repair via nonhomologous end joining. Located in site of double-strand break. Biomarker of nephroblastoma. [provided by Alliance of Genome Resources, Apr 2022]