Odahviing is a Red Dragon that becomes an ally of Dovahkiin against Alduin in the main questline of Skyrim. In the quest "The Fallen", Dovahkiin will trap Odahviing in Dragonsreach and interrogate him. Odahviing will tell Dovahkiin that Alduin has gone to Skuldafn, and upon further interrogation will say that that Skuldafn is inaccessible without wings and that he will fly you there upon his release. After taking you to Skuldafn he will fly away, leaving Dovahkiin with entering Sovngarde as the only way out of Skuldafn.
After defeating Alduin and returning from Sovngarde, Odahviing can be called to help you in battles. He will use fire breath and standard dragon melee attacks. Odahviing cannot be defeated and won't fly away until all nearby enemies are dead. This makes him a great use for players of any level. He will fly around the Throat of the World awaiting to be called. His name in dragon language means "Winged Snow Hunter"