Oh Disco Elysium makes this *fun* - I consider the actual given name of the protagonist a spoiler, but uh, AO3 does not. On Twitter, where it's harder to discuss the game without worrying about spoiling newbies, I often see initials for him (HDB) or an alternate name you can get semi-jokingly. (Semi-, because the entire *game* is about whether or not said protagonist can truly start over again. The alternate name is ostentatious and ridiculous, but also, well, given that theme...)

Lieutenant Kim Kitsuragi, on the other hand, is always precisely himself. In-canon pet names are 'Kimothy' and 'Kimkim', and I tend towards the latter. Fandom has also decided on the pet name 'Kitsy' for a particular AU that I also love. The narrative often refers to him as 'the lieutenant', though interestingly, I usually see the rank come up in uh, NSFW/power dynamic contexts. (This is one of the first times I've been in a police/detective based canon - is this rank kink common elsewhere?)