Thorton and Marley were made for each other. There's a lot of subtext indicating they are in love.
Marley doesn't like talking to the player because she wants to be loyal to her boyfriend and doesn't want to be tempted by the player. She is a goth and therefore bisexual so the player's gender doesn't affect her attraction. She's trying to find Shaymin as a gift for him because she knows he studies Pokemon and doesn't know how to show her love. She shows up in the battle zone in Platinum because she is staying at Thorton's house.
Thorton doesn't talk much in Platinum because he's busy with Marley. Marley says that she hates being left alone, so Thorton developed a device to teleport to her whenever she needs him. He even styled his hair like her's as a way of branding himself as her property.