I'm assuming you mean the novel 'The Last Letter' by Rebecca Yarros. I've seen this one pop up a lot in romance circles lately. It's a single POV book, so the main character is definitely Ella MacMillan. She's a single mother, the anchor of the story, and her perspective is what we get from start to finish. The whole plot hinges on her receiving that final letter from her brother's army buddy, 'Chaos'.
Now, the other lead is Beckett Gentry. He's 'Chaos', the Special Forces soldier who writes to Ella without revealing his true identity. Their connection through letters forms the emotional spine of the story. Even though we don't get chapters from his viewpoint, his character is fully fleshed out through his actions and the letters themselves.
The kids, Maisie and Colt, are pretty major secondary characters too, especially Maisie given her illness. Her storyline is central to the conflict and the choices Ella has to make. So, main cast is Ella, Beckett, and the kids, with Ella firmly in the driver's seat of the narrative.