If you're looking for manga similar to Semi-Eternal Happy End, you might like these titles. All recommendations are made by Anime-Planet users like you!
All murdered souls arrive in limbo at the Gate of Grudges, where the Gatekeeper, Izuko, presents them with a choice: they may pass through the Gate into Heaven; they may cling to life on Earth and remain a ghost there forever; or, they may haunt and kill one person in exchange for eternal damnation. As each spirit passes before the Izuko she is bound to explain these options and impartially accept their decision; what will each choose? Peace, or spending an eternity in Hell?
A person who has been slighted in some way is given an important choice, where making the wrong decision could have disastrous effects.
Both have the same somber tone, and are likely to appeal to the same audience.
While not explicitly related in plot, these two do indeed have a similar feel and a similar idea - a person must make one or more important decisions that can have powerful consequences. Each is fairly bleak and dark and should appeal to the same type of person.
Yucy and Yudy are two conjoined twins who have been together since birth. However, as the nutrients in their bodies distribute unevenly, Yucy is beautiful yet slow-minded while Yudy is intelligent and horrifically ugly. Yudy is torn between her love for her sister and her hate for having to care for Yucy; can the two continue to co-exist?