Vita Sexualis

One day while Shirou was delivering a secret weapon to his colleague, he met Amane Mao, a super strong girl who is capable of lifting cars. Before they knew it, an ERROR, a monster that struck Old Tokyo 14 years ago, reappeared and started on a rampage. While he was arguing with his colleague over the use of the super weapon to save civilians, his colleague was blown away and died. Faced with death, Shirou had no choice but to combine with Mao with the super weapon and destroy the ERROR. ( from AnimeSigma)