If you're looking for manga similar to Cassette Girl, you might like these titles.
It ain't easy being the savior of Mankind when you're still in high school, but Hoichi Kano will have to make do. Luckily, he has a super-cool integrated gunsuit at his disposal — just the ticket to take on the monstrous mechanical horrors of a pesky group of all-powerful alien invaders. But eventually, fire must be fought with fire, and luckily Earth may have the biggest beat-stick in the playground: the massive, anti-matter-powered robot, Exaxxion. And of course, all it needs is someone crazy enough to pilot it. Oh, Hoichi...
In a far and distant future, men and women have become mortal enemies. Each living on seperate planets, an interstellar war is being fought between the two very different peoples. Hibiki, a mere 3rd class worker, finds himself in the middle of a huge space battle, facing the nemesis of mankind: Womankind!
An expansion of the Strike Witches series, World Witches - Contrail of Witches details the short stories of various witches from areas of the European front of the Neuroi War.
In the near future, after a comet strike has devastated the Earth, the Eidrons and their Shadows appeared. When one of these Shadows - giant human-like robots - attacks a passenger plane, young and unsuspecting Leefa gets pulled into a secret war. Accused of summoning the Shadow, Leefa must prove her innocence - only to discover that she really is an Eidron! The Eidrons want to assert their supremacy over the human race, and Leefa must choose sides quickly. Which path will she follow?
Ryotaro is a boy who loves robots and lives out in the country with his three sisters but he's headed to Tokyo for high school. Before leaving, he runs across a humanoid military robot and has a fateful encounter with its pilot, a cute teenage girl who calls herself La Vie en Rose. Ryotaro will almost certainly meet her again in Tokyo, considering his family connection to the production of these humanoid robots--his father worked with them.
The year is 1944, and the war against the Neuroi is over. The 501st Joint Fighter Wing, also known as the Strike Witches, have accomplished their goal of protecting Britannia from the alien invaders. Now that the battle is done, the Witches disband and decide to go their separate ways for some much-needed R&R. As the girls enjoy downtime in the warstricken lands of Gallia and the deserts of Africa, a new danger lurks. A mysterious alien/ human hybrid foretells of another impending invasion. Can the Strike Witches regroup in time to defeat this latest evil threat?