Zepar was one of the nine pillars that administrate the Blast Furnace. After the defeat of Goetia, it was one of the four Demon Gods that escaped from the Time Temple.
Salomon: The Grand Time Temple
Following the initial defeat of the Blast Furnace pillars under Naberius, Zepar voices complaints towards Control for them being pressured by the Heroic Spirits, blaming a flaw or mistake in the design of their plan.
At some point, he had a last meeting with Raum where they were debating about the means to save mankind. After knowing about his plan of making use of an outer existence in order to bring salvation upon man, Zepar states how foolish his error is because his answer had been proven to have failed 14000 years ago.
SE.RA.PH. - Deep-Sea Cyber-Nirvana, Epic of Remnant: Extra
After escaping the Time Temple, Zepar sought to help humanity realize their mental potential. However because he lost his body, he needed to seek refuge somewhere to regain his strength. He eventually found Seraphix. There he found the therapist Kiara Sessyoin, and possessed her to avoid Chaldea's detection. He then used her to eventually effectively gain control of Seraphix, and despite her efforts to stay in control she realized that her personality was slowly being eroded. Using his knowledge of parallel worlds, Zepar discovered another version of Kiara. Impressed by her, he linked the two Kiaras together, and used her body to convert Seraphix into SE.RA.PH and give it form. Afterwards though, Kiara (now completely controlled by her CCC self) plunged the Seraphix staff into mass hysteria to Zepar's disgust. She then seduced the personnel to open the planetarium, where she placed 128 Master candidates in the Coffins found there to summon 128 Servants, which killed them; however, their magic circuits remained active so they could be used to summon new Servants indefinitely. She took control and ruled SE.RA.PH from the behind the scenes, all the while claiming she was doing it for Zepar. It was at this point Zepar realized that Kiara has overtaken him. Kiara refused to create a Singularity according to Zepar's plan, and instead sought to become a Singularity herself. Regulated to her pinky finger, Zepar was eventually released and quickly faded away as little more than a mindless thrall begging for Kiara's forgiveness.