Actually, she's Lady Larken, not Lady Larkin. Larkin seems like a feminine name to me, despite the more masculine -in ending, because of the whole lark thing.
I wouldn't name my kid this or anything, but it looks like a terrific name for a character in a fantasy book, and maybe even a good name for a band, if you think about it.
Larkin was the name Lauren Olamina gave her child in the book Parable of the Talents. Unfortunately, Larkin was kidnapped at the age 2 months old and renamed Asha.
In the book Baby by Patricia MacLachlan (of Sarah, Plain and Tall fame), the main character and narrator is named Larkin. I had never heard this name until I read that book, but I like it a lot. It sounds more feminine to me, probably because the book character is a girl.
Larkin is also the name of Larkin Groves, a female character from the American TV series "Invasion" (2005-2006), played by American actress Lisa Sheridan (b. 1974).