Tad, the "marked character"

[Donna McCreary]

In a letter to Francis Bicknell Carpenter, dated November 15, 1865, Mary Lincoln wrote a few lines about Tad.

"Dear little Taddie! was named, for my husband's father, Thomas Lincoln - no T - for a middle name was nicknamed, Taddie, by his loving Father. Taddie - is learning to be as diligent in his studies, as he used to be at play in the W.H. he appears to be rapidly making up, for the great amount of time, he lost in W - As you are aware, he was always a marked character. (punctuation as in Turner's Life and Letters.)

What do you think Mary meant when she referred to Tad as a "marked character?" I have read several thoughts including those that refer to Tad's education, speech patterns, or even a possible mental challenge. Is it one of these theories, or something else?

What I find interesting is Mary's choice of the word "character" instead of "child."