A Reaper (Blade II) has such an accelerated metabolism that if it doesn't feed on blood almost constantly, they will starve to death.
Mimi Force (Blue Bloods) and all the other vampires are blessed with being forever skinny and perfect.
Soul Bees such as Q-Bee (Darkstalkers) have such a fast metabolism that if they don't eat something within 36 hours, they will starve to death.
L (Death Note) eats nothing but sweets, yet never gains any excess weight due to him metabolizing it for his immense brain power.
Mello (Death Note) eats nothing but chocolate bars, and just like his predecessor, L, he never gains any excess weight.
Near (Death Note) starts eating chocolate bars as a homage to Mello, and just like L and Mello, Near did not seem to gain any excess weight.
Saiyans (Dragon Ball) tend to have a higher metabolism due to their battle-inclined natures.
Son Goku (Dragon Ball series) possesses such an incredibly fast metabolism that he once ate a 50-course meal in one sitting without any adverse effects on his body.
Stephanie Powell (No Ordinary Family) has an accelerated metabolism.
Though possibly for comedic effects or bioabsorbing, Monkey D. Luffy (One Piece) never seems to gain any permanent weight despite his voracious appetite.
Despite being gluttonous, Jewelry Bonney (One Piece) doesn't gain a single pound or have any adverse effects on her body. It's quite possibly due to her Devil Fruit powers giving her the metabolism of a child.
Sephie Michaela Deviluke (To Love-Ru Darkness) has an accelerated metabolism to maintain her beautiful look.