I honestly don't remember who my first LJRP journal was for, mainly because I don't remember what I did first, actual OCs or esoteric often-minor characters who I could make up almost anything about.
This was aimed towards apping to Milliways Bar and they don't allow OCs, at least outside of special circumstances like "canon characters' kids born in play", so the first OC journals were essentially for practice. But I don't remember if that came first.
The first canon characters I can recall making journals for are the narrator from Hanna Is Not A Boy's Name and a joint account for Zimmy and Gamma from Gunnerkrigg Court. Not sure who actually came first. There was also a Lone Wanderer from Fallout 3 but I think that was a bit later.
Ah, I also did that! Super minor canon characters for games that didn't allow OCs. Or really obscure ones that nobody would know. I was also at Milliways for a while