Tannin is the Dragon who trained Issei in the Underworld, helping him to become stronger and attain his Balance Breaker. Issei and Tannin have such a close relationship that Issei calls him "Uncle Tannin". The two are really good friends and have fought numerous battles with each other. He was, at one time, a Dragon King and knew Ddraig before he was sealed. When Issei died, he was sad, along with many others, but rejoiced when he came back to life.
Tannin knew Ddraig from before the time the latter was sealed and seems to be on good terms with him.
Tannin gets easily angered by Midgardsormr due to his lazy attitude.
They share a friendly relationship with each other, considering how Tannin took Crom Cruach under his care without a second thought despite the latter being an Evil Dragon and the most powerful one at that.
Not much is known about their inter actions, other than the fact both them know each other since ancient times. As Tannin awares that his powers of magics, spells and sorcery are highly very dangerous, to the point that he warns Vali to become extremely careful towards him.