Early demos of the game featured Senua with a different design and personality; appearing more confident and less vulnerable compared to the final version. The demo also shows that the story was likely going to revolve around Senua rescuing her mother from Hel, instead of her lover, Dillion.

When working with experts, the team realized that "quite often, the illness comes not from the symptoms, but from the stigma, isolation, and mistreatment that comes about from the rest of society". This led the team to make Senua's father, Zynbel, and the way he abused and isolated her during her youth the real nature of the Darkness which haunts her.

Since the team only had one camera, they opted to make all the cutscenes be presented as one single long take, to make the viewer feel like "an observer, like a presence watching Senua throughout".

The team, researching Celtic culture and the Celts' views on mental disorder, found out that they used the term "gelt" for a man or woman who had been driven mad by a curse, grief, or the trauma of a battle, and that the gelt would take to a life in the woods in search of penance, punishment, and purgatory; the team decided to make Senua a gelt, who had left her home in exile for those reasons.

The main inspiration of Senua's character was the Iceni queen Boudica, while her name came from Senuna, a Celtic goddess long lost to historians but re-discovered in 2002, whose name was at first read incorrectly as Senua.

Ninja Theory's video editor, Melina Juergens, was used as a stand-in for Senua while they perfected their motion capture techniques. They also based Senua's physical appearance on hers and eventually came to the conclusion that she could play the character in the finalized product. Game director and lead writer, Tameem Antoniades, was confident that she could pull off the role despite her having no acting experience.

Druth was based on the Irish Celt Findan who was enslaved by the Norsemen and eventually escaped in the 8th century. He was given a backstory similar to Findan's and was named after another word used by the Celts to describe mental disorder, "druth", meaning "fool" or "one who utters the words of gods".

After the first combat encounter, the game will display the message:

The dark rot will grow each time you fail
If the rot reaches Senua's head, her quest is over
And all progress will be lost

Spoiler:This Implies that if Senua dies too many times, the player's save file will be deleted and the game will reset. This message is a ruse, as Senua can die any number of times without the game resetting and the rot only grows artificially at fixed points in the story.

Early builds of the game show directional swinging and ducking were options during combat. These features were not in the game's final version.