Arisu ran at Keith. Her full form allowed for greater speed, and a smaller target. She was generally a lot harder to hit. Plus Arisu could only try possession if she was close enough, and she doubted that she could get that close in her other forms. 

Possession, Arisu's only other method of attack. It didn't have a 100% success rate, but even if she failed it was a distraction. If she succeeded Arisu would get control over Keith's body. And she was pretty sure that that would count as a victory.