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Petyr Baelish - Littlefinger
I was a bird,
unheard-of and small
His symbol is a mockingbird
In love with the big fingerling,
Catelyn Tully (Tully sigil = trout)
I staked it all
Bit by the wolf
I lost it all
In pain and humiliation,
I began to loathe them all
In quiet, I began to
plan their fall.
Challenged Brandon Stark to a duel and lost
Helped by falcon
I began to build my halls
Betrayed and sick,
worse fate did him befall
Jon Arryn (Arryn sigil = falcon) helped him (made him Master of Coin), but Petyr betrayed him (convinced Jons wife, Lysa, to poison him)
And so it began
All their downfall
Wolves, stags all
suffered their fall
Escape I did
before the lion's bawl
Many characters died (Wolves=Starks, Stags=Baratheons, Lions=Lannisters), but Petyr escaped from Lannisters with Sansa Stark
I returned to my home
without any care of it all
Returned to his home (The Fingers)
To save the pup
I made the fish fall
To save Sansa, he pushed Lysa (born Tully -> fish) to her death
Complete it is not
the tale of my gall
Though I should tell you
I mean to have it all
He is very ambitious and ruthless
I suppose even the title is a hint (naym -> Petyr)
edit: the hidden lines
But given Richard's style
I might meet my end before nightfall
George Raymond Richard Martin is known for "ending" a lot of characters so P.B. might end up dead before we expect it (Nightfall -> Long Night -> rise of the Others)