Yona is a travelling priest and spellcaster. Being a member of religious order she travels the realms of steamland in hunt of spirits and daemons, protecting and rendering justice.
In SteamFantasy world the machinery is powered by steam and spirits thus the machines have 'souls' inside them. Spellcasters like Yona could catch the free spirits or fight the awaken ones.
ps. I did my best trying to introduce this character to you without spoiling any details.