I dreamed I was a butterfly, flitting around in the sky; then I awoke. Now I wonder: Am I a man who dreamt of being a butterfly, or am I a butterfly dreaming that I am a man? (Zhuangzi)
I don't know which book you're referring to but regarding my agathodaimon a.k.a. HGA, I wouldn't want to bare my soul to another entity, fictional or not, that is basically foreign to me. Such entities always come from another pantheon or system, with their distinct personality, backstory, powers, etc., they just don't jibe with my idea of "myself made perfect" or however you want to see your guardian angel. Such a being might be a patron in the way demonolators cultivate them, a teacher, a source of power and inspiration, a protector and whatnot but always as an entity your are basically inferior to, that you revere and maybe even construct an altar for (how would an altar built for Iron Man look like, I wonder). The relationship would be one of veneration while my agathodaimon (which I hope to attain some day) and I are essentially equals. I like see him/her/it as my buddy, as myself who has successfully mastered all the difficulties I had to and have to contend with in my life and transcended my flawed personality. Me as my own Ipsissimus, so to speak.
Nevertheless, the idea has some attraction if you think invocation, not daimon. I think an already created fictional entity from a work of literature, movie etc. would have even more power because it would represent a popular archetype (think hero here, for example) shared by millions of people, just like a deity of old. Such an archetype would be already highly dynamic and colorful because there is always a story as well as adventures attached to it while you have to work harder creating your own spirit from scratch that is even remotely credible and powerful.
So I might invoke John Wick some day but I wouldn't want him as my Holy Guardian Angel.