( The 'd' in this name is pronounced like 'd' in 'den' .)
Hidimba was a demon living in the forests on the shores of the river Ganga . His sister was named Hidimba too. ( The only difference between these two names is the gender which is simply clear in Samskritham ( spelt as Hidimbah and Hidimbaa )from their ending vowels, but that cannot be seen in the English transliteration where both the names are spelt alike.)
When the Paandavas managed to escape from the wax-house attempt and reached a forest by midnight for rest, all fell asleep except Bheema who stood guard alone. The demon Hidimba living around that area felt the scent of human existence and sent his sister Hidimba to kill them and cook for him. She went to them in guise of a human woman,but, Hidimba fell for Bheema in the very first sight itself and disclosed her identity to him with a fond request to accept her as wife. . Hidimba grew impatient over the delay in his siter's return and went to her only to see her joined with them. He grew furious and fought fiercely with Bheema . Bheema killed him and upon the advice of his mother Kunthi and elder brotherYudhishthira , married Hidimba . A son named ghatothkacha was born to them. Hidimba never appeared in the story again.
Ghatothkacha was of immensely helpful to the Paandavas . During their travel through the Gandhamaadana hills, he carried them safe to the destination. In the war too, he fought bravely for 14 days defeating many heroes and killing two prominent demons on the kourava side Alambusa and Jata. He made a devastating attack on the Kouravas side in the 14 th day's night-fight with the aid of his demon followers and his magical powers. All feared total destruction. Karna was persuaded to use his Shakthi ( given by Indra ) againt him to save himself and others, risking his winning chances against Arjuna .
Ghatothkacha's role was very prominent in the story. He was used as a shield to Arjuna from Karna . But for him, the Paandavas would have lost their dear Arjuna