Yakumo is a character from the game Nu: Carnival, voiced by Manaka Sawa.
Official description from the game’s website:

Descended from the famous Great Serpent of legend, Yakumo has inherited a vast amount of power. When he was a child, he was found by an elderly couple who brought him back to their village and raised him as their own. Kindhearted and polite, he lives a cautious life, ever fearful of the huge power he possesses going out of control. If Yakumo gets angry, you better run.