The answer is RITA MORENO
The corresponding square is ELECTRIC COMPANY
   by Brian Tivol

The letter reads as follows:

  Hello, my dear nieces and nephews.

  Although you may be grieving, I hope that you will still try to lift
  your spirits and enjoy the finer things in life.  Take some time out
  of your busy schedule and do something fun.  Perhaps you should go to
  a party on a Friday night.

  Who knows?  Not only could you end up having a good time, but you
  might also find an important clue encoded in exactly the same way as
  this message is.  It might even be four words long, with the first and
  third words being proper nouns.

  The law firm I use generally has some sort of get together on Fridays,
  and I'm sure you would be welcome there.  In fact, I bet that if you
  missed the party (or just missed something at the party), you could
  make an appointment and bring your own party to them.  My only advice
  is that you remain pleasant and avoid businesslike questions such as
  "Where is that secret message?" and "Now that I've found the message,
  may I please be excused to go back to my drudgery?"

                                          Yours,
                                          Rich Uncle Pennybags

No messages were spelled out at the party, but some fairly obnoxious
background music was playing.  (Even considering that Dan is on our
team, it's obnoxious.  (ed.: though Ray is now a fan of AFI)) Like the
song "How To Be A Millionaire" (by ABC), each of the songs on our
party mix were by bands with three letters in their names.

  KWS: "Please Don't Go"
  KLF: "What Time Is Love?"
  LEN: "Steal My Sunshine"
  REM: "It's The End Of The World As We Know It (And I Feel Fine)"
  AFI: "A Winter's Tale" 
  TLC: "No Scrubs"

Decoding "KWSKLFLENREMAFITLC" and putting in the appropriate spaces
gives the clue "Tuptim in Lang's movie".  Tuptim is a character in
"The King and I", and in the film directed by Walter Lang, she was
portrayed by RITA MORENO.