Think of Mrs. Sen as the opposite of a freezing New England winter. In fact, her place is the opposite of the "tiny beach house where [Eliot, the boy she babysits] and his mother lived year-round…already cold" in September (MS 12). Her place is "warm, sometimes too warm" (MS 12).
Like her apartment, Mrs. Sen is much more inviting than what Eliot's used to. She's maternal. She does things like make sure Eliot doesn't come near her when she's chopping food with her huge mega-blade from India, even though she's super-skilled with the blade. She meets him at the school bus with snacks.
Mrs. Sen is also lively and passionate, which is why the red sari she wears on one of their outings fits her personality perfectly.
In other words? Mrs. Sen totally doesn't fit in with her New England setting. She feels it too, which is why she's constantly reminiscing about her life in India and missing her extended family and her chauffeur.
You read right. Her chauffeur. Mrs. Sen comes from one of the upper classes in India, which is part of the reason why she doesn't like the modest, slightly shabby apartment near the university. It's also the reason why she's doesn't want to learn to drive.
The other reason? American traffic scares her. But in other ways, Mrs. Sen is determined. She doesn't let anything stand in between her and her great love—fresh fish. She's the kind of person who's used to getting what she wants (which is why she's able to get Mr. Sen to pick up the fish at first), even if it means doing something she hates, like driving.
We understand why Eliot likes Mrs. Sen. She's everything his mother isn't (see: "Character Roles" for more on this comparison).