Grief

Anytime the annotation [possibly] is shown, it indicates a feature that may be implemented in the series. 
Grief is a topic covered in more than one book in the Face of God series, though primarily through Stacey's character arc in dealing with losing her best friend Rose Sanchez. Grief is characterized as one of The Silent, a non sentient yet personified group of auras that are the arch enemies in the series. The Silent are come to exist spontaneously in the absence of many factors: for example, they are the darkness that results from the absence of light (for disambiguation, see The Silent). 
[Spoilers Following for Heart, Silence, and other books in the Stacey Monners saga as well as the books in the God Saga of the series]
The Heart begins Stacey's story with her first year at Central Michigan University as she copes with her friend's sudden death in a car accident. Stacey has many dreams with crinkly skinned, alien like figures that she learns to be The Silent. The first book in the series ends with her arc at am ambiguous end, yet with the idea of life itself is moving on. 
Hand goes through the simple life in Thomas Mans, a pathologist who discovers he has the ability to bring judgement on anyone he chooses. He becomes a vigilante and starts a dark character arc. After losing his grandmother to cancer, the concept of grief is explored. Though not the main concept of the book, the idea of grief as loss that you live with is present until the book's end.
Heart's Hand shows Stacey's continued arc through college as she learns more about her ability, and works with Eli Laffe is under the table ways (due to Eli being tracked by the United States government for his ability) to understand what their purpose is with the Silent. They are actively tracked by the government while also meeting various characters, such as Luke. The plot takes place fifteen years after Hand, and Thomas emerges, without any explanation to the reader, with a small army of individuals who he has embued with his own power: to bring judgement on anyone they wish. In this book, the narrative is third-person, yet it is colored with Stacey's own view of her world and how she sees so many things as connected to her failure with Rose's death. This fourth wall voice is a [possible] feature seen in each of the 16 main characters' arcs, where each one of the 16 Meyers-Briggs type indicator personality types is attributed to a character, and the particular personality for said character taints the voice of the writing style in each book within their saga. (See Meyers-Briggs and the Face of God Characters). Because the narrative is written with this "INFP coloration," the idea of grief is explored in Stacey's thoughts; she has moved on, but the situation is still apart of her thoughts. In this book, Stacey and Thomas interact, lending to the name, Heart's Hand.
Heart of Hearts or Ace of Hearts takes place after Stacey is out of college, and her life continues at a normal pace as she goes in and out of two different jobs at small accounting businesses. In this book, Thomas is gaining traction with his army of followers, and Eli works in the background, the focus being on Stacey and her life as it goes on. Stacey learns the real cause of Rose's death was government intervention, and her wound is reopened. The idea of grief as a tool to learn how to grow as herself is presented in this book. The connection between Stacey and Thomas is also hinted at: being that Stacey ability is to bring life, and Thomas is to execute the actions necessary to facilitate that life. These concepts--of a purpose for each characters ability--is explored later in the series (See The Main idea of The Face of God).
In conclusion, at the series' end, grief is expressed as a way to build each character and as something that exists but you can become whole after it has passed.