Koihime†Musou: Sangokuden no Gaim [On Hold]

Kamen Rider Gaim (仮面ライダー鎧武) x Koihime†Musou (恋姫†無双) Crossover

The story tells about Yuuki Kusanagi (草薙 悠来), a modern-day Japanese student and the school idol of St. Francesca Academy as well as a hardcore fan of Kamen Rider, especially the newest Kamen Rider series: Kamen Rider Gaim (仮面ライダー鎧武) due to its Samurai and Sengoku Era theme.

During the museum trip, he was sent back in time to ancient China alongside his younger sister and two childhood friends when they've trying to stop a mysterious thief from stealing an ancient Chinese mirror, but ended up shattering the mirror in the process.

Yuuki Kusanagi found himself in the world of Romance of the Three Kingdoms, where every major figures had become beautiful girls. Not only that, he gained the ability to transform into Kamen Rider Gaim (仮面ライダー鎧武).

With his new comrades on his side and as the new Kamen Rider Gaim and taking his role as the first Messenger of Heaven (天の御使い), Yuuki seeks to unite the land and bring peace to ancient China and stopping the Helheim Forest (ヘルヘイムの森) from devouring the world.

Note: Kamen Rider Gaim and Koihime†Musou have officially belonged to Toei Corporation and BaseSon respectively. I didn't own both series.