Questions that don't deserve their own topic

[Ivogoji]

Game Knuckles: mysterious past is genuinely mysterious
Fleetway Knuckles: he was born in Tikal's time period and has been on the Floating Island for like 8000 years
Archie Knuckles: got retcon'd too many times to say anything concrete about him
OVA Knuckles: is a wandering treasure hunter with no island to protect
Sonic Underground Knuckles: had a thing for Sonic's sister, has a pet dinosaur, his grandpa is still alive so he's not the last Echidna
Sonic X Knuckles: is a weaker, dumber version of the one from the games
Sonic Boom Knuckles: Knuckles in name only

Game Shadow: isn't who you think he is, I'll tell you that right now
Archie Shadow: just sort of showed up one day, the Archie adapt of the SA2 doesn't really explain things
Sonic X Shadow: just like game Shadow but the Space Colony Ark is in Chris's dimension so his backstory makes no sense

Others: don't know

[Toasty_Tyrant]

They actually explained some Adventure 2 stuff in issue 2 of Sonic Universe.

Spoiler:

And some of the Black Arns stuff in issue 171

Spoiler: