Most important person in Lincoln's life

[Gene C]

Juan asked about the single most influential person in forming character and outlook, and your expanding it to key figures.
To answer Juan's question, there is no doubt in my mind it was his parents (all three), with his mother being the most influential.
I know this because Lincoln said so regarding his mother. (I don't recall him differentiating between his mother and step-mother, so the choice is yours)

However, history has always hinted at, but slighted, one who falls into the key figures category in his Springfield years, so I'm glad you brought that up. He's right in front of your nose.

Fido