Role in the Film
In her childhood, Martha was the only one who was ever nice to the Grinch as he had a beard despite being only 8. She flirted with him by saying that she only loved the color of red and green, then caressed his face with her hand.
This surprised The Grinch who decided to make a gift for her in reciprocation for her feelings to which he shared.
The next day, when the Grinch presented his gift to Martha, the other classmates laughed and bullied him for shaving in an attempt to impress Martha.
Before Martha could defend him from the ridicule, he left in a rage and has lived on a mountain ever since and she had not seen him since, despite carrying the same feelings for him for many years.
As years passed by, Martha grew up to become a sultry and mature woman. She's also became a rival with Cindy's mom Betty Lou Who who challenge to win the light decorating contest each year which Martha wins each year (mostly due to Martha's monetary standing and the mayor's feelings for her).
Later she's seen telling Cindy what she knew about The Grinch whom she known since childhood, but also inadvertently confesses to having had a "crush" on him.
At the celebration, Martha is seen unimpressed by The Mayor's behavior. As Grinch arrives, she is seen watching him from the distance and cheering him, but hiding her feelings without being noticed.
At the moment of Gift Pass On, Augustus takes the opportunity to propose to Martha, she was speechless that Augustus asked her to marry him which makes her feel uneasy since she's in love with The Grinch.
Before she answers the proposal, she is interrupted by The Grinch who starts ranting about the commercial and frivolous side of Christmas.
She decides to think about Augustus's proposal until the next day. By Christmas morning, Martha watches how Augustus publicly blames Cindy Lou for what happened, even if the blame was his.
When Lou defends Cindy, she listens Lou's speech about the true meaning of Christmas, is touched by it and realizes that Christmas is also about spending time with your loved ones. So, she finally make a choice after hearing that speech.
By the end, Martha decide to call off the engagement and gives Augustus back the ring when the Grinch reforms after revealing her true feelings for The Grinch which makes The Grinch happy.
Later, she was last seen during the song "Welcome Christmas" and at The Grinch's home where he's cutting the feast and they both flirting for a while. It seem that they're in relationship after they smiles at each other before the movie was over.