Alexander Koryagin:

AK> "He thinks we need a miracle," Sticky said in a bleak
AK> voice."  "Well, optimism has never been his strong
AK> suit," said Kate.

I think you have an extra quote there!

AK> "Suit" -- is it pertain to playing cards?

You ask whether the term pertains, or is pertinent, to card
games.  I think `suit' there denotes general perseverence in
an effort, but the dict.org begs to differ:

long suit
    (a) (Card Playing) the suit[8] of which a player has the
        largest number of cards in his hand; as, his long
        suit was clubs, but his partner insisted on making
        hearts trumps.. Hence: [fig.] that quality or
        capability which is a person's best asset; as, we
        could see from the mess in his room that neatness was
        not his long suit.

strong suit same as long suit,
    (b) . "I think our strong suit is that we can score from
        both the perimeter and the post." --Bill Disbrow
        (basketball coach) 1998. "Rigid ideological
        consistency has never been a strong suit of the Whole
        Earth Catalogue." --Bruce Sterling (The Hacker
        Crackdown, 1994)
        [1913 Webster +PJC]

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