Also known as: 惑星のさみだれ, the_lucifer_and_the_biscuit_hammer
惑星のさみだれ, lit. Summer Rain of the Stars, or The Lucifer and Biscuit Hammer
A 2005-10 manga by Mizukami Satoshi, serialized in Shounen Gahosha's Young King OURs.
The series follows Amamiya Yuuhi, a college student who wakes up one morning to find a talking lizard who tells him that he is needed to help save the world by finding and protecting the princess and then fighting the enemy trying to destroy the world. Events take an odd turn, as Yuuhi has no interest in getting involved and the princess' goal for saving the world is far from heroic.