As for DR0, it was released before DR2, and was meant to be a prequel to the first game while setting up important things in the second game. DR Kirigiri is very good, I’ve heard, same with DR Togami, though I don’t know how far the translations are for both games, but they give characterization to those named characters.
Another Episode gives Toko and Hagakure (at least in the little novel unlocked after you beat the game for Hagakure) some needed characterization, but it’s highly recommended you play it after DR2.
There’s the Killer Killer Dr Gaiden novels, which feature characters from the DR3 anime and go into another story entirely I’m not sure of (though the thing about cloning is cool).
Once you beat DR2, you can unlock a novel of sorta called DR IF, which is a What If for the first game and is the closest thing to a look at the personality of a spoiler character from the first game (apart from appearances in DR0 and the DR3 anime).
As for DRV3, I have literally no clue as to how stuff goes down besides who died (not because I’m avoiding spoilers, but because the game’s story sounds Pandora Hearts levels of confusing), but I heard it’s getting an English translation soon, and has been met with mixed results.
There’s also manga based on the games, some from POVs of different characters in the games.
Quite frankly, I’m disappointed by the lack of character figures (besides the little coin ones) for the series besides the most popular characters. There aren’t even any Figmas. What if I want Naegi to knock Komaeda the heck out?