WARNING: MULTIPLE TV TROPES LINKS
In my three-part series, The Ragnarǫk Cycle I plan having my protagonist and deuteragonist to undergo a character arc. The protagonist, Joseph Norton to put it lightly... is kind of a dick, who thinks only of himself and is an existential nihilist to boot. Throughout the trilogy, Joseph is called out on his flaws multiple times by various different characters (including his younger brother, his commanding officer, the deuteragonist herself and a major villain) and it's eventually revealed via a series of dream sequences/flashbacks that they stem from an abusive childhood, spending his early teenage years living on the streets and being betrayed by former friends.
Unlike most portrayals of nihilism in the ever-expanding realm of fiction, which basically amount to "Oh dear me, the world is so terrible, life sucks, pain, pain, pain!", Joseph's nihilistic views revolve around the belief that his life is meaningless, because he doesn't know what his purpose in life is. Both Joseph and the deuteragonist clash repeatedly due to their conflicting ideologies and differing personalities over the course of the story, but slowly become more respectful and accepting of one another.
I plan on having Joseph to grow and develop in a different way in each instalment, e.g in Surge (the first book), he becomes more selfless. In Soulbound (the second book), he learns how to open his heart to those closest to him and Infinitum (the third and final book) sees Joseph finally abandon his nihilistic worldview.
The deuteragonist is heavily implied to be Jeanne d'Arc (yes, THAT Jeanne d'Arc) and serves as Joesph's conscience by calling him out on his less-than-heroic decisions. Much like Joesph, I plan having different aspects of Jeanne's personality evolve in each book as per her character arc. In Surge, she eventually becomes respecting of Joseph and accepting of his ideals.
In Soulbound its shown that Jeanne uses her faith in God to shield herself from an overwhelming feeling of self-loathing and guilt born from the acts that she committed during the Hundred Year War. A large part of Soulbound revolves Jeanne confronting herself and coming terms with what she did. In Infinitum, it is suggested that Jeanne secretly wants to die due to the fact that she exists within an era that she's unfamiliar and all of her loved ones are dead. The only reasons why she can't commit suicide is that it greatly conflicts with her Catholic beliefs and she has her soul contained within an ancient artifact called a "Divine Tool", rending her immortal.
The only problem is that I have is that I'm unsure how to set these arcs into motion and execute them properly, as I'm afraid that it seem that Joseph and Jeanne stumble into becoming better characters in a very contrived manner. I am uncertaint on how to approach these concepts, and I will be very happy if you can help me out with it.