Unlike in the real world, death isn't always final in the world of anime. Just because your favorite character bit the dust doesn't mean you'll have to mourn them forever... and on the flip side, just because that character you hate is gone doesn't mean you won't see them again.
Anime characters who came back from the dead tend to appear in fantasy-themed anime - you'll find them in shows like Inuyasha and One Piece, but you're not going to find them in realistic, slice-of-life shows. Sometimes, as in Konosuba and Saga of Tanya the Evil, the character is reborn in a completely different world with new responsibilities, while on the other end of that spectrum, they simply carry on as normal.
Some argue that revivals cheapen the impact of a character's demise, while others feel that it adds intrigue and excitement to a series. Depending on the quality of the series, either one is possible.