Hajime does not immediately know what has just jumped out of the tree, save that it is attacking. He's faster than a normal human would be, much faster.

And because it's not in his nature simply to run away from a fight, he reaches for some of his power, goes for the partial demonic transformation he'd managed to achieve. He looks like a human suddenly calling insectoid armor into existence, which isn't too much of a stretch given the nature of the way he's tweaking reality to meet his needs. He's going to guard for now, because what even is happening here.