All I can think of is Boston Corbett because he met Lincoln at a reception when his NY unit arrived in D.C. at the onset in 1861, to guard the city from a suspected invasion from Virginia. However, I don't think that Booth knew Corbett
I'm trying to think of a character that would socially priviledged enough to attend a public reception with the President and the only one I can come up with is Mudd.
"There are few subjects that ignite more casual, uninformed bigotry and condescension from elites in this nation more than Dixie - Jonah Goldberg"
If this is a side character in the assassination story, but not directly related to the assassination, then I'm going to vote for John Ellsler, one of Booth's partners in the oil fields. Ellsler was into the theater business, was well-respected, and was the manager of a musical enterprise in Cleveland (I think) that might have put him in the presence of Lincoln.