Mona

   ∧_∧   / ̄ ̄ ̄ ̄
  ( ´∀`) < Look who's talking!
  (    )   \____ 
  | | |
  (__)_)

Mona (モナー) is the most guy of 2chn, if not one of the top guys of all time. He's representative of a generic Japanese person/2channel user. His friends are Moralar, Giko, and Shii.

  ∧_∧
 (    )
 ( O   )
 │ │ │
 (___(__)

This is the rear end of Mona. Note that his tail is small and round, like a bear's.

Mona's Origins

         / ̄ ̄ ̄ ̄ ̄\
          |  You guys    |
   ∩_∩  |  have too   |
  (´ー`) <  much spare  |
  (   )  |  time.      |
   | | |   |           |
  (___)__)   \_____/

Mona is thought to have originated on either Ayashii World or Amezou, which were both precursors to 2ch. He looked more bearlike then, and his catchphrase was "おまえらも暇な奴らだなぁ""omae-ra mo himana yatsurada nā", translating to "You guys have too much spare time", a saying used to remind internet users to go touch grass. This original variant of Mona is sometimes called Great Ancestor Mona (ご先祖モナー).

Then someone replied to a Mona with another Mona saying "マエモナー"/"omae mona", meaning, "So do you!" or "Look who's talking!" That instantly became Mona's new catchphrase.

Derivatives:

Round-Eared Mona

   ∩_∩  / ̄ ̄ ̄ ̄ ̄
  ( ´∀`)< Let's Enjoy!
  (    )  \_____
  | | |
  (__)_)

Round-Eared Monas

Sources: