Leo doesn't fully know what's happened when he hears Donnie cry out, but it's enough to trigger something in him; a protective instinct stronger than usually that only comes out when he feels his family is in some kind of mortal peril. He can't see Donnie properly in this thick forest of zombies, so he doesn't know how he is, so he doesn't know what the yelling was for, so-

Gone are the puns and the wisecracks and the quips. Leo is almost eerily silent as he starts hacking faster through the zombies that are between him and his brother, with a ferocious but efficient violence. He can't let something happen to him - not again.