There's not all that much to say about Marina. She's almost always in the background, never interjecting much or doing enough to change the course of the play. Her role in the family is nyanya, which is kind of like a nanny or housekeeper. She's considered almost part of the family, though, and shouldn't be thought of as just a maid.
In fact, Marina has her opinions and isn't afraid to speak them, even if no one really pays attention to her. She complains a bit about the way that Serebryakov lives his life:
MARINA: [shaking her head] What a way to live! The Professor gets up at noon, and the samovar has been going all morning, waiting for him. Before they came we always had dinner before one o'clock, like people everywhere else, but with them here it's after six. At night the Professor reads and writes, and suddenly he rings after one in the morning…I ask you, gentlemen. For tea! (1.66-71)
Marina is obviously not afraid to speak her mind, but she doesn't really put her money where he mouth is. Even as Serebryakov ruins her precious schedule, she does whatever he asks, serving him tea whenever he wants. She even babies him from time to time:
MARINA: [going to Serebryakov, affectionately] Master, what's the matter? Does it hurt? My own legs ache and ache. [...] [Kisses Serebryakov on the shoulder.] Master, come to bed… Come, dear… I'll make you some lime tea and warm up your feet… I'll say a prayer to God for you… (2.112-22)
By comparing her own aches and pains to Serebryakov's, she almost puts them on the same level. And the way she kisses him on the shoulder is a sign of respect, but also shows familiarity. This tells us that Marina is not just an employee; she's a part of the family.
It also shows, though, that she's only part of the family if she keeps on nurturing. She's not supposed to criticize anybody or rock the boat. Even if she has some useful opinions, she's really just there to make people feel good and make sure their days are running smoothly. It's another way in which the familiar, everyday, humdrum reality of the play just keeps on going. Nobody wants to change, and nobody wants to hear what they're doing wrong.