first occurence of non repeated character

FirstNonRepeated[str_] := With[{chars = str // Characters},
  Select[
    {#, Count[chars, #]} & /@ chars,
    #[[2]] == 1 &
    ][[1, 1]]
  ]

(* now lets test this *)
FirstNonRepeated /@ {"teeter", "stress", "test"}

(* will output: *)
{"r", "t", "e"}


(* this is how you "read", in normal language, the code above:  *)

(* create a function of a string... split the str up into its characters
FirstNonRepeated[str_] := With[{chars = str // Characters},
  (* select the elements ... *)
  Select[
    (* ... from the list of characters with their according count of occurence ... *)
    {#, Count[chars, #]} & /@ chars,
    (* ... where the count is 1 *)
    #[[2]] == 1 &
    ][[1, 1]] (* ... from this pick the the first character *)
  ]