Apparently some popular pairs are McCree & Genji and Pharah & Mercy. I have no clue what the reasoning for the pairs is but some funny/cute fanarts/comics pop up on the subreddit all the time. There's also ones of 76 and Mercy acting like parents taking care of other heroes.
Honestly Blizzard has done an amazing job IMO with really making these characters unique and interesting, especially when all the in-game mediums for lore are essentially just quick snippets of dialogue. The cinematics help a lot, of course, but blizzard cinematics have always been top notch.
EDIT: Also, I have been playing SUPER casually on PC, and still placed rank 61, which is apparently still in the top ~5% of players on PC. I've found that playing in 3/4 mans is ideal for ranking up, as anything less leaves you with not enough players to consistently perform everything you need in-game and anything more means you will match 6 man teams more frequently (which needless to say are sometimes difficult to deal with without a full party). You can always gamble with searching 6 man but you essentially just have to be better than the other party all the time, which quickly becomes difficult because the higher your rank the less 6 mans there are, and the ones that are left are really good.
I also have decided to be a Mercy main, because I'm really good at being aware of where everyone on the map is. Situational awareness is pretty much the only skill you need as a Mercy player, 5/7 would recommend.
2/2 of the BR Leech Octagon Hall of Fame