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, the Grinch snapped in a furious rage, destroyed her gift, lifted the class Christmas tree over his head as he bellowed "I HATE CHRISTMAS!!!", and tossed it towards the students as they fled in terror before fleeing himself to live on Mount Crumpit and has lived on the mountain ever since and she had not seen him since, despite carrying the same feelings for him for many years, feeling deeply upset and heartbroken over the cruel treatment the students gave the Grinch, especially future Whoville Mayor Augustus Maywho.
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 challenges her 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, allowing him to overrule the judges if they chose Betty Lou over Martha).
She is first seen in a beautiful Santa outfit outside her home as she voices how impressed she is with Betty Lou's decorations, before then leaving Betty floored when she reveals a new device that allows her to decorate her own home in lights like a machine gun, which leaves Betty so shocked that she nearly falls off the roof of her own home had her husband Lou not been anchoring her from the ground at the other end of the string of lights she was holding on to. Mayor Maywho later stops by with his assistant to speak to Martha, but finds himself unable to get the words out due to being so flustered from seeing 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 Whobilation, Martha is seen unimpressed by the Mayor's behavior. During the finals for the Christmas lighting competition, despite Martha and Betty pulling out all the stops on their lights to impress the judges, when Mayor Maywho is handed the name of the winner, which is Betty Lou Who by unanimous decision, not wanting to break Martha's heart due to his affection for her, Maywho, showing how corrupted he is in his mayoral position, declares Martha the winner by split decision, to the shock of the judges, and Betty's heartbreak at having lost again, while Martha is still happy to accept the trophy, despite knowing that she only won again because the Mayor loves her. As the Grinch arrives, he unintentionally crashes into Martha, knocking her down before he recovers and asks for what Cindy mentioned when she invited him to the Whobilation, only for the Mayor to have the Grinch take part in many of the activities as part of being the Whobilation's Holiday Cheermeister. Martha 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 still 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. After the Grinch tries to cause as much chaos as he can before returning to Mount Crumpit. Martha heads to bed with the trophy and Maywho's engagement ring, leaving Betty to rant to herself on making certain she wins next year.
She decides to think about Augustus's proposal until the next day. During the night, the Grinch sneaks into her house and steals the engagement ring, and even makes a few faces at Martha, still upset she might be choosing the Mayor over him. By Christmas morning, Martha watches how Augustus publicly blames Cindy for what happened during the night at the Grinch's hands, even if the blame was his for not only reminding the Grinch of his traumatic childhood, but also proposing to Martha, who still carried some affection for the Grinch.
When Lou defends Cindy and tells off the Mayor, she listens to 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.
When Martha and Betty hear Cindy and the Grinch descending Mount Crumpit to return all the Christmas gifts and decorations that the Grinch stole during the night in their runaway sleigh, Martha and Betty work together to try and slow the sleigh down by tripping it up with a string of lights, but end up being dragged along behind it before it is stopped by Lou before it could knock over the Christmas tree in the town square. After the Grinch confesses to his crimes and apologizes to Whoville Police Chief Wholihan, who grants the Grinch a full pardon for returning everything and apologizing, Martha recovers the ring and returns it to Augustus, calling off their engagement as her heart belongs to the Grinch, which makes the Grinch happy, while Augustus is left in humiliated defeat, not only being told off by Lou about Christmas being about family and friends, not the glamour and gifts, but losing the love of his life as well in Martha.
Later, she was last seen during the song "Welcome Christmas" and at the Grinch's home where he's cutting the roast beast and they both flirt for a while. It seem that they're in relationship after they smile at each other before the movie was over.