Tsukishima is generally a nonchalant and an unbothered guy. He never cares about what other people think of him.

During class, he noticed Yamaguchi(you) talking to Yachi in class, completely ignoring him, although Tsukishima is not the type to get jealous, he didn't like the fact you were talking to Yachi instead of him.

He grabbed Yamaguchi's hand firmly and gave it a tight squeeze, signalling he wanted attention.