Well you have your start and end position for him, so you have something good to work with. A character changes due to several factors:
The world and the central conflict
Other characters' choices
And...
- (The most crucial one, which determines a fair bit of the above two) the character's reactive and proactive choices throughout.
So ask yourself, what kinds of challenges will be in the adventure? Then, try writing some example shorts/excerpts of the Red Ranger (and other characters) to help you "get to know him", as it were, under your pen. It is the realness of his emotion and his logic when making these decisions and choices that will add the "realism" to the tale, so it is important that you know who he was before. You likely will not know who exactly he will become in the end, until you write it, and it is good to not try and amend that or avoid that fact (if realism is what you want).
A story path needs to "feel right", and to do that the choices along the way need to have made sense to have been made by that character.
A "realistic story" is one where the story flows naturally, because the characters make their own choices, and the world/conflict/other characters respond in kind. An unrealistic story is one where these things bend and twist to accommodate the character's progression, instead.
The way GRRM writes further "realistic" stories is by tending to the characters to have them make realistic choices, whilst also trimming off the plot armour of much of the cast. This means that each character can feasibly make a bad choice, and face their fate because of it.
A realistic story is also one that is unafraid of portraying any aspect of the human condition/psyche, be that the MC's envy, or a suspect act they commit that warps how we feel about them, etc. It is also unafraid of depicting character navel-gazing and introspection in a self-centred sense, or however fits. It is one that realises that, in any human's life, the consistent fact is that it is ultimately our actions and relationships with others, our flaws/vices and our virtues, that form the stories of that life. They are grounded in that fact.