tenacious
Function: adjective
continuing despite difficulties, opposition, or discouragement -- see PERSISTENT

per·se·ver·ance
Pronunciation: "p&r-s&-'vir-&n(t)sFunction: noun: the action or condition or an instance of persevering : STEADFASTN

relentless
Function: adjective
showing no signs of slackening or yielding in one's purpose -- see UNYIELDING 1