The pronoun associations you're looking for—old men use washi, tomboys use boku and so on—are more a trope of fiction than of real life (to an extent, even things like "women's speech" are more a prescription or ideal than an accurate description of real-world speech patterns). Luckily, someone has been studying exactly those tropes: Satoshi Kinsui, who calls it 役割{やくわり}語{ご} or "role speech". Take a look at this example (from this Japan Foundation page):
Can you connect the characters above to the speech patterns below?:
- おれは、この町が大好きだぜ。
- あたしは、この町が大好きなのよ。
- わしは、この町が大好きなんじゃ。
- ぼくは、この町が大好きさ。
- わたくしは、この町が大好きですわ。
It's not the kind of thing they teach you in Japanese classes, and again, it's not representative of real life; but if you've been reading manga or playing videogames, it will sound incredibly familiar and quite self-evident.
So I recommend you look for Kinsui's work, starting from wikipedia or his website. If you have access to Japanese sources, try to get some of the material published on yakuwari-go. Consulting Kinsui's 〈役割語〉小辞典 I can find, for example, that the pronoun nanji:
- Was originally a respectful second-person pronoun.
- By Heian it was already in use for people of equal or lower standing.
- After the feudal era, widely used for lower standing.
- In Taketori Monogatari, used by the Emperor to address the Old Man (he includes the exact quotation).
- After Meiji, becomes associated with the voice of God in religious texts, especially the Christian Bible. (Also with sample quotations.) Thought to be chosen by translators due to sounding classical at the time. Examples can also be found of people using it to address God, so perhaps at first more a matter of elevated textual style than yakuwari.
- In novels, found connected specifically to Bible quotations in Nakaishi's 『死刑囚と其裁判長』 (1922), to things like marriage ceremonies etc.
- In manga, found in the literal voice of God in Black Jack (1974).
- In many fantasy/RPG-like settings, used by characters in a divine or cosmic position of absolute authority, such as creator-gods. (There's an illustration from the big reveal at the end of CLAMP's Rayearth; a character who just revealed themselves to be a creator-god starts using the pronoun nanji-ra).
- Also used by characters such as sages or saints whose role is to guide the protagonist, especially when extremely old. (The yakuwari pronoun is therefore tagged 老人語 and 神様語).
- Often associated with Western-style fantasy, but not exclusively; there are examples of it being used by Eastern-type Gods and Buddhas.
- Finally, used by characters who hold some sort of divine or otherwise special power, in things like magical incantations etc. The example is magical girl Sakura, of card-capturing fame: 「汝のあるべき姿に戻れ。クロウカード！」
I'm posting a summary of the entire entry because I want to give a sense of how extensive is Kinsui in laying out fictional patterns of language and their associations. If you're interested in the topic, I highly recommend this author. See here for an introduction which lists some pronouns; if you're unsure about the connotations of any of them, you could try looking'em up on nicopedia, or just general web searches (the keyword 役割語 is your friend).
That being said, the most natural way of learning these tropes is to expose yourself to them. If you want to write fantasy, why not binge on fantasy novels or manga? High-school students interacting with elves etc. is an entire subgenre (including e.g. the aforementioned Rayearth), and if you like light novels, there are more of them than anyone could possibly handle. Go read stuff you like, and take notes on which pronouns are used by which kind of character when addressing who—and not only pronouns, but also verb inflections and such. You'll not be procrastinating, it's research!
