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 series) tend to have a higher metabolism due to their battle-inclined natures...
...Son Goku possesses such an incredibly fast metabolism that he once ate a 50-course meal in one sitting without any adverse effects on his body...
Majin Buu (Dragon Ball series) of all forms eats nothing but sweets, and copious amounts per serving, yet never did they suffer any adverse effects (Fat Buu is already fat due to absorbing Grand Kaioshin instead of eating too much).
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.
Toriko (Toriko) never gets overweight…
…because his of Gourmet Cells.
A-Train (The Boys)
Despite having an almost bottomless appetite, Dean Winchester (Supernatural) never seems to gain any extra or permanent fat.