This is Frisk, the Protagonist from UnderTale.

Useful links:

http://undertale.com/

http://undertale.wikia.com/wiki/Player.

Also, i have not completed this game yet. So please do not post spoilers in the comments. Thanks :)

Edit: I was actually calling him "Protagonist" to avoid spoilers for people, but it seems people already know it. Fixed.

ＩＦ ＹＯＵ ＤＯＷＮＶＯＴＥ, ＰＬＥＡＳＥ ＬＥＴ ＭＥ ＫＮＯＷ ＷＨＹ ＳＯ Ｉ ＣＡＮ ＦＩＸ ＩＴ :)