The Strongest Dull Prince’s Secret Battle for the Throne [Licensed by Kadokawa]
Title: The Strongest Dull Prince’s Secret Battle for the Throne
Romaji: Saikyou Degarashi Ouji no An’yaku Teii Arasoi
The protagonist is nicknamed 'the dull prince' due to his laziness and incompetence, or so he wants others to believe. This way he can ditch his responsibilities to go out and play around as a mysterious masked adventurer. But with the battle for the throne heating up he realises his loved ones will end up dead once one of his power-hungry siblings wins the title of crown prince/princess, so he decides to help his younger twin brother become the next emperor.
Nobody's going to be paying attention to him since he's so 'dull', so he can easily conduct schemes in the background. He also has the assistance of the empire's strongest adventurer - himself!
It's similar to Altina in that there's a bunch of siblings fighting to become emperor, but this series focuses more on political maneuvering rather than battlefield tactics (although the characters do occasionally go out and fight things). The protagonist is interesting in that he willingly lowers his reputation for the sake of his own freedom, and because it's easier to help his brother that way.
There's currently 4 volumes and a manga adaptation being serialized.
This series has just been licensed by Kadokawa themselves.
It doesn't show here but there were at least two additional topics started suggesting this title that I'm aware of, which got locked down after receiving supporting comments with links directing here; at least I hope it was here, otherwise there's yet another topic still active for this title...