There were a lot of things that happened when someone ran towards you. Your fight or flight instincts took over and Magnus always overwhelmingly had the urge to run away. Even with the amount of training he had done with his floor mates and fighting all the various Norse monstrosities, he wanted to run. He didn't understand people who liked fighting.
Still, he wanted to do this for Percy. And that was the only reason why he didn't scream and run away like a five year-old.
When Percy got close enough, Magnus couldn't help the small shout as he brought up Jack to defend himself - it was a shout that inwardly said why was this a good idea?
Jack seemed to have other plans, though, and instead of thrusting up to meet Riptide, he flung back, causing Magnus to fly back a foot.
"Jack, what the hell man."
"Sorry, she's just so pretty, I don't want to hurt her!"
Magnus swore. "Jack, this is not the time."
Jack made some sort of huff sound and Magnus' blade stopped glowing. Jack was refusing to help. Great. At least Magnus knew how to swordfight... though Percy was almost definitely better.
Magnus swung back at Percy and this time Jack didn't interfere.