Information from the Official Site
Martha is one of the many leashed-dogs that stumble upon Lucky and like the rest of her friends, quickly see their new companion as the leader of their group. She is a Newfoundland breed which are born with webbed-feet, making them rather strong swimmers. This soon comes in handy, as when she and her friends soon set out to find a new home, she saves Bruno from a certain death from drowning in the river.
She feels a connection with the river dog and greatly respects it.
Martha is a compassionate dog who cares for the makeshift pack. She towers over the rest of the pack but is actually the most gentle.