If you're looking for anime similar to Zatch Bell! Side Story: Friends, you might like these titles.
Anos Voldigord was a tyrannical Demon King that eradicated humans, spirits, and even the gods, but became bored of eternal warfare and reincarnated with dreams of a peaceful world. However, what awaited him in reincarnation after 2000 years were descendants who became too weak after being accustomed to peace, and all sorts of magic that deteriorated to the extreme. Anos enters Demon King Academy that gathers and educates those who are viewed as the reincarnation of the Demon King, but the academy could not see through his true powers and ends up branding him as a misfit!
When Princess Syalis is abducted and taken to the demon king Twilight's castle as a hostage, neither the castle's demonic ruler nor his subjects realize the trouble they've brought upon themselves. The deceptively adorable princess is actually a holy terror who has no difficulty sneaking out of her cell, destroying property, and murdering the creatures she comes across all for the single-minded purpose of getting a perfect night's sleep. The dumb-as-rocks hero Dawner has been sent to rescue Syalis and bring her home, but his mission seems unlikely to succeed anytime soon; and in the meantime, the demons must deal with their ruthless royal prisoner constantly causing an uproar in the castle. While the princess rests in peaceful slumber, her captors may never sleep again!
The greatest demon lord in all of history, Varvatos, has reigned for several millennia as an absolute ruler. Now, there's only one thing left for him to do... enjoy the life of an average commoner! When he's reincarnated as a villager in a non-descript town, everything seems to be going as he hoped but there's just one slight problem-even when he holds back, Varvatos is still too strong! Soon, rumors about him spread and he's approached by all sorts of men, women, and assailants! The quiet life he's always dreamed of seems far away indeed...
After saving the world, the strongest hero Leo became someone who is not wanted in the peaceful human world. He was too strong. Exiled, he seeks a job at the Demon King's Army, which he had defeated and needs to be rebuilt. The Army had many problems: too much work, financial troubles, etc. Leo starts to make things better using his power. Leo encounters Echidna again, and asks her why she invaded the human world.