The lights are on
One of the unique things about Devil Survivor Overclocked is that who dies varies on your playthroughs. Here's how you save everyone who can die. With the exception of Haru the final events you attend to save people are marked as battles. The times of death are given to you via Laplace Mail (or in Gin's case, Gin himself)
Mandatory set up: Talk to Kaido and Mari to find out about the bloodless murders, this is important to saving Mari and Keisuke. Later, when asked to find Mari's bag, make sure you find it. Finding her bag takes no time so don't confuse it with something else.
Haru: Attend her event on the fourth day. When she attempts suicide inform her that Demons were in Tokyo before the game.
Mari: Make sure you give Mari her bag (either directly or via Kaido).Go to her event on the fifth day and Whatever you do, make sure she deals the killing blow to Kudlak.
Keisuke: Give Mari's bag to Kaido (see above) and attend his event on the fifth day. Either fight him and kill Yama without killing Keisuke or agree to stay out of his way*
**Gin: Attend his event on the sixth day and help him fight the Shomonkai
* choosing this option will cause Mari to leave you and later fight you with help from Kaido
** Gin is only in danger of death if you give him the information you gathered at his request. However doing so is needed to unlock his route.