Underachiever

[Renato]

Person who fails to [achieve] his or her [potential]. Used often when refering to [students].

[Magaret]

A person who does not [perform] to ones [expectations].
A person who [disappoints].

[Luis Fok]

The opposite of:
[overachiever]
44 up, 15 down

Someone who is sooo much better than you that it pisses you off.
I hate that fucking overachiever!
by Blah [Aug] 8, 2003 [email it]

[Tina]

someone who is less successful than they should be at school or at work