The word steady comes from trhe root word stead (meaning a place, spot, or station) combined with the suffix -y.  It may also compare to old continental forms like Middle Dutch or Middle Low German stēdig.  It appeared in the 1520s to mean firmly fixed in place.  It traces back to Old English and Germanic roots meaning a standing place or position. 

According to the Oxford English Dictionary (OED), the adjective was first used in the 14th century, and the verb steady first appears in English in the mid-1500s (specifically 1530) in the writings of scholar and teacher John Palsgrave.  The adverb steady was first recorded around 1605 (though related forms like steadily appeared slightly earlier in 1540).  The noun steady was first recorded in 1792 in the writings of George Cartwright.  In the 1520s the adjective was used to describe things "firmly fixed in place or station," replacing older words like steadfast for physical objects.  In the 1530s the Verb was adopted as an action word meaning to make or keep something firm and free from shaking.  The meaning was expanded over time to describe even rhythms, calm behaviors, and regular habits, as well as romantic partners ("go steady") in the 20th century.  

A steady or steadfast character provides grounding, stability, and clear moral compass in a story.  They anchor the chaotic plot, highlight the flaws of changing characters around them, and prove the core message of the theme.  

Why a Steady Character Matters:  
Anchoring the Plot: Gives the reader a safe, reliable point of view when everything else in the story world changes or falls apart.  
Proving the Theme: Acts as living proof that the character's starting truth or value is right, testing it against heavy pressure without breaking.   
Guiding Other Characters: Shows a strong example that forces dynamic characters to face their own flaws and decide whether to change.  
Building Trust: Makes the narrative feel safe and predictable in a good way, so the audience can relax and focus on external conflicts.