Wild

Wild is a boar-like creature who lives in Purgatory. He spends his life searching for his younger brother, Mild (also known as Hawk) and fighting the Demon King.

He was born in the Purgatory. He used to take care of his younger brother, Mild, singing lullabies that he adored. However, his brother was kidnapped by the Demon King shortly after he was born and sent to another world.

For millions of years, Wild looked for his younger brother, coming to face the Demon King on many occasions, but always being defeated.

Some time after, he coincidentally met Meliodas and Ban in purgatory and decided to join force with them to defeat the Demon King so as to escape out of purgatory.

(Source: Nanatsu no Taizai Wiki)