So according to Can someone break down かのじょ and かれ for me? it seems that 彼 and 彼女 are words that only came to mean "he"/"she" due to the influence of the West. But I'd like to know a bit more about how those third person pronouns came to mean boyfriend/girlfriend. Some preliminary research indicates 徳川夢声 might be behind the reason for 彼氏 and 彼女, but if any additional light/specifics could be shed on the top that'd be great
Let's look at these two in turn.
According to Shogakukan's 国語大辞典【こくごだいじてん】 (KDJ) entry here, 彼氏 initially meant just "that person", derived as 彼【かれ】 + honorific person suffix 氏【し】. Apparently the honorific was used a bit ironically, and the term was actually intended as a slight pejorative. If the entry is anything to go by, it appears in texts starting from 1929, in one of 徳川【とくがわ】無声【むせい】's works.
A bit later in 1934 in a piece by 永井【ながい】荷風【かふう】, we see usage in the sense of "boyfriend, male lover". This appears to be a straightforward extension of the meaning "fellow, guy" in usage patterns like "she's got a guy, that's her fellow", where the "boyfriend" sense arises out of context. Compare German usage, where Mann literally just means "man", but in the context of talking about a woman's relationships, it also means "husband". (The English term man is also used the same way, albeit in a more informal speech register.)
I suspect that both terms were in common use before either publication appeared.
Here's the KDJ entry link. Use in the "she" sense as a female third-person pronoun shows up in 1885.
Then the "girlfriend, female lover" sense shows up later, and is recorded in the 1931 ウルトラモダン辞典【じてん】. I don't see any usage examples from that time, but again, my hunch is that this too was an extension of the earlier "she" meaning: "he's got a girl". Compare German usage, where Frau literally means "woman", but in the context of talking about a man's relationships, it also means "wife".
Once again, these meanings were probably around for some time before they were put to print.