RE: The Witcher 3: Wild Hunt | I haven't played an RPG in a very long time.

You'll also come to a point later on where you'll meet the emperor and he'll ask you a few questions about certain characters.

Letho is the Antigonist of the second game. A witcher from the school of the cat.

Roche is a part of the Tamerian Special Forces, and is a side character in the second game. At a certain point you can either decide to go with him or Ivoreth.

Ivoreth is a Scoia'tael ( A group of Elves and Dwarves who are a "terrorist" group)

Roche and Ivoreth play big parts in the second game. If you have the chance to play TW2, you should take it as IMO TW3 is a really poor game to jump into if it's your first witcher game, as it referencing quite a bit of stuff that happens in the previous games.