Tannin is the Dragon Issei trained with in the Underworld and helped him become strong 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.
Tannin seemed to know Ddraig for some time before the series and are on good terms
Tannin is easily angered by Midgardsormr due to his lazy attitude.
They shared a friendly relationships to each other. As Tannin took Crom Cruach under his care without a second thought although he is an Evil Dragon and the most powerful of them all.