Martin is so close to the James family. He has likely been with them since Annie and Hallie were born. He is a sommeliea, which he demonstrates by choosing a "lovely choice of wine" one night for dinner. Elizabeth describes him as a "lovable brother", which speaks to his close relationship with the James family.
It is revealed he likes swimming, as at the Stafford, he explains he's "going for a dip". He is also very close to Annie, as he accompanied her to Camp Walden, and has nicknamed her "Old Bean". They even made up a secret handshake together, consisting of eighteen moves.
At the end of the film, Martin falls in love with Chessy and they get engaged.