Enma - The Monkey King is Hiruzen's powerful summon and long-time companion, his body and tail are covered by white fur which protrude from his sleeves and pants. He has long unkempt white hair that reaches his back - accompanying long sideburns with a goatee. He wears a black suit with mesh armour underneath, over which he wears a sleeveless kimono-shirt with white fur trimmings, and markings reminiscent of tiger stripes on it, which is held closed by a red sash. Enma also wears a Konoha forehead protector. (Cited from the fandom page)

Unlike other summons - Enma has more autonomy over his thoughts and actions - going as far as being critical of Hiruzen for not killing Orochimaru in their initial encounter for example. But despite as such, Enma still loyally served as a close partner to him.