Who Sir James Martin is
Sir James Martin is a rich and rather foolish young man whose proposed marriage matters to Lady Susan's plans. He is courting Miss Manwaring when Lady Susan attracts his attention at Langford.
Sir James Martin and the others in the story
Lady Susan chooses Sir James as a husband for her daughter, Frederica Vernon, without caring about Frederica's wishes. She sees a wealthy match as a way to settle her daughter's future and relieve herself of a burden.
Frederica is shy and frightened, and she does not want to marry Sir James. She fears her mother too much to say so, leaving Lady Susan's wishes at odds with her own.
Questions
Who is Sir James Martin in Lady Susan?
Sir James Martin is a wealthy, rather foolish young man whom Lady Susan selects as a possible husband for Frederica.
Does Frederica want to marry Sir James Martin?
No, Frederica does not want him, but she is afraid to tell her mother.
Why does Lady Susan choose Sir James Martin?
Lady Susan chooses him for his wealth, treating the match as a solution to her daughter's future rather than considering her feelings.