X Blade

Four years ago, a large scale disaster in Tokyo claimed two million lives. The remaining thirteen districts of the city were isolated into the Tokyo Barricade Zone. Morisaki Harumi, an unruly kid with great fighting strength, is his family's sole survivor from the incident.

One day, in a traffic accident, Haru gets pulled into a world of violence and meets Mana, the girl who turned herself into an ancient sword.

Not to be confused with the action-adventure game X-Blades

Tropes used in X Blade include:

Action Girl (Despite spending most battles as a sword, Mana is more than capable of holding her own in a fight)
Battle Couple (pretty much every Anathemea and their wielder to various extents)
Bullet Time (Haru has the ability to perceive movements as much slower than they are. However, he can't move any faster, meaning if he can't tell where it will go, he can't avoid an attack. Whether it's a trait of his own, or a result of Kuushirou is debatable.)
Conveniently an Orphan (Haru lost his immediate family four years prior.)
Empathic Weapon (The Anathemea or 'Guardian Blades')
Grand Theft Me (Kuushirou, Mana's previous wielder, inhabits Haru's body. More than once he's fully taken over. Unfortunately for Haru, Mana sees this as the preferential outcome.)
Hot Chick with a Sword (Several of them. The rest ARE swords)
Loves My Alter Ego (Mana is very much still in love with Kuushirou, and openly prefers him to Haru)
Naked First Impression (Mana, when she first wakes up. Also Naked on Arrival, since it's the first time the audience sees her as well.)
Nebulous Evil Organisation (Hollyhock)
Powered by a Forsaken Child (The Anathema were once people, sacrificed to create the weapons. Worth mentioning most were voluntary)
What Year Is This? (Mana, once she wakes up fully, having been asleep for 360 years)