Dead characters are transferred into Doomheim corporation [666], where they will be unavailable. Technically reviving a dead character is possible, but needs a damn good reason.
In character, dead character is dead. Unless you hand wave that the character's DNA and earlier mind state have been backed up. (There are some hints in the game that the "cloning accident" mentioned in the character dead mail for the corp is no accident.) Naturally, ignoring the game, we've seen some "dead" characters who, in fact, have retired from capsuleering.