Klaus stood up fully. This was the choice he made when he decided to absorb the energy instead.
He couldn't imagine how Tessa would feel if she woke up and her father was dead. It was better for his true self to be exposed than Sir Mateo dying, he just wouldn't be able to look at Tessa's face if that had happened.
Klaus strengthened his body and took one step forward.
He doesn't want Tessa to see it. He doesn't want Tessa to see his true self.
What would Tessa do if he suddenly grew bigger and taller with the face of a monster.
He doesn't know how she would feel, but he knows that he wouldn't be able to look at her anymore. He could only talk and play with Tessa because Tessa hadn't seen the true him. Once Tessa sees Klaus Marshall, not Klaus Walker, everything would definitely not be the same again. No way she would like him. She would be afraid and she would distance herself from him.
'Humans are always afraid of the unknown'