Histone demethylase that specifically demethylates Lys-9 of histone H3, thereby playing a role in histone code. Does not demethylate histone H3 Lys-4, H3 Lys-27, H3 Lys-36 nor H4 Lys-20. Only able to demethylate trimethylated H3 Lys-9, with a weaker activity than KDM4A, KDM4C and KDM4D. Demethylation of Lys residue generates formaldehyde and succinate.