Smartass (Umnik / Умник) is a character who appears only in S.T.A.L.K.E.R.: Shadow of Chernobyl.
A short bio, that was being omitted from final version of S.T.A.L.K.E.R.: Shadow of Chernobyl game describes him as follows: 
Shadow of Chernobyl
He is a Veteran Loner, who can be found in the small camp north of the Freedom Base in the Army Warehouses. He is equipped with a Stalker suit and armed with an AK-74 with a PSO scope. Baldy gives a job requiring his assassination in exchange for a rare Night Star artifact. Baldy wants him dead after he stole the artifacts they found together and sold them to Barkeep and lied to him when he claimed that the artifacts simply disappeared.