When he fights he sometimes opens his jaws and his teeth get sharper.
If he gets his hands on 3-7 meter kyojins he bites them in half or rips them to shreds (another reason why they call him nightmare)
There was one time he lost control over his form(the first time he ever lost controll) and Anthony fought him. They way he fought is the other reason they call him a fighter type.
Note: since he is a 20 meter he bites the nape of 15 meter ones and swallows it as an act of revenge. He does it as pay back for them eating soildiers.