Lord Boros is an alien overlord that fights with Saitama and manages to almost be on par with him in terms of strength. 

20 years before the start of the series, while Boros was laying waste throughout the universe looking for an opponent that would be able to stand against him, he met a prophet that revealed to him that in a far off planet he would finally find a strong opponent that would be able to fight on par with him, giving Boros the enjoyable fight he had always desired

Since he has incredible powers, Boros is arrogant and prideful. He roams the galaxy looking for worthy opponents, for his love of battles.

Boros should sound powerful and intimidating. You can even add voice effects to make him sound more "Alien-like".