Although the world of Across Five Aprils is small, the novel boasts a diverse cast of characters. The protagonist, Jethro, is first introduced on a spring day during potato planting season. Hunt writes, "The world seemed a good place to him that morning," but it soon becomes clear that Jethro's cheerfulness persists despite the hardships on the farm and in the world, not because of them. Jethro is the youngest child of Ellen and Matthew Creighton, and four of his siblings have passed away. Additionally, his oldest brother, who ventured to California years ago, has not been heard from since. The four siblings buried in Walnut Hill Graveyard cast a quiet shadow over the living family members, serving as a poignant reminder of life's fragility in an era when child mortality rates were alarmingly high.

At the story's outset in 1861, Jethro is not yet ten years old, but he is aware of the uproar surrounding Abraham Lincoln's 1860 presidential election and "of tariffs, of slave states and free ones, of a violent old man named John Brown and... of states seceding from the Union." However, Jethro is more captivated by the impending civil war that these issues will ignite. On the evening of the novel's opening day, news arrives at the farm that Fort Sumter has been attacked.

In the following months, significant changes occur on the farm. Jethro's brother Tom and cousin Eb enlist to fight for the Union. By autumn, Jethro's brother Bill departs to join the Confederacy, confiding only in Jethro before he leaves. Letters from the soldiers begin to arrive at the farm. One letter from Tom challenges the romanticized view of war, stating, "You tell Jeth that bein' a soljer ain't much." Jethro's brother John and his schoolteacher and future brother-in-law Shadrach Yale, whom Jethro deeply misses, are the next to enlist.

Jethro's mother Ellen and his sister Jenny stand out as some of the novel's most resilient characters. Ellen, like many women of her social standing in the mid-19th century, has devoted her life to her family and remains uneducated and illiterate. While she is intrigued by the complex events of the time, her comprehension is limited. Occasionally, she closes her eyes briefly, as if "to hide her exasperation from the world."

Jenny, in contrast, reflects both her mother's strengths and those of the emerging generation. She is educated, and at fourteen, she is in love with Shadrach. Despite her youth, she is mature and capable of managing a household nearly as well as her mother. At sixteen, she travels to Washington, accompanied by newspaper editor Ross Milton, to marry the injured Shadrach. Her letters from Washington vividly depict the war's impact on the capital city.

A third female character, John's wife Nancy, initially appears somewhat elusive. Jethro detects a lack of warmth and affection in her, but over time, during their shared periods of anxiety, they grow closer. It is she who delivers the heartbreaking news of President Lincoln's assassination to Jethro.

Numerous intriguing characters outside the Creighton family add depth to the story. Ross Milton motivates Jethro to pursue his education, sparking a passion for language in an environment where many people speak in less refined English. The importance of education, although a minor theme, is strengthened by its connection to two admirable characters, Milton and Shadrach.

The Burdow family, involved in the previous death of Jethro's sister Mary, introduces a sinister element. However, Dave Burdow assists the family by warning Matthew Creighton of threats against Jethro that he has overheard. Consequently, the youngest Creighton learns that benevolent impulses can exist in those who seem malevolent.

Across Five Aprils portrays the grim realities of war, offering little to glorify. Several secondary themes support this perspective, such as the war's divisive impact on families and communities. Jethro's moral struggle over Eb's desertion highlights the complex conflicts that emerge. While Jethro believes desertion is wrong, he also comprehends Eb's reasons. The cowardly federal registrars who harass the families of deserters scarcely inspire respect or loyalty. Jethro's actions reflect his sound judgment and his growth throughout these challenging years.