These heroes do not always have to be the opposite of heroes (known as villains) to be knights of cerebus, they just appear in comical titles that usually goes against the comical rules, or they appear in a title that is strongly focused on seriousness, drama, and less humor.
In other terms, Knight of Cerebus is a rare type of hero who appears in media where there is usually comical or incompetent heroes and the hero quickly goes against those rules.
For a good guy qualify in this category, the hero should provide intentions that are very serious or stern than any other hero that has appeared in a film, book, TV Show, or a video game, and having the presence to perform actions that's usually not meant to be funny.
Good examples for this are Shadow the Hedgehog from the Sonic the Hedgehog franchise and Mewtwo from Pokémon. Other examples, like Lelouch vi Britannia from Code Geass, Jellal Fernandes from Fairy Tail, Kurumi Tokisaki from Date A Live and Kyoko Sakura from Puella Magi Madoka Magica are seen as more competent, dark and ruthless than the rest of the heroes. ◾IMPORTANT: This type of hero is not always necessary to be a one-shot hero, as it can also be applied in recurring hero in special episodes noticed in a much more serious way than how you're used to (Often occurs when a character undergoes a personality change that is called "Flanderization"). ◾NOTE: a Knight of Cerebus can be good-hearted bastard in a setting where it normally doesn't make sense - not simply a "anti-hero".