After the return of Kouji, Tetsuya was frightened of Professor Kabuto replaced him with his biological son and discarded him, and he was constantly angry, irritable and snapping at everyone -specially Kouji-. In one of those times he accused Jun of wanting replacing him with Kouji, too. She slapped him. Deservedly so.
Tetsuya is arguably Mazinger trilogy's walking generator of this. Especialy when he does his Big Damn Heroes.
The third episode is properly named "Honoo Guts! The Wild Venus". In that episode Venus A made its debut fighting Evil Spirit War Beast Zugar, and Jun proved she was a better pilot than Sayaka and Venus A was very superior to previous Fem Bots. She effectively battled the Mykene War Beast, and she even got the upper hand sometimes. It is possible she would have even won if Boss would not got in the way CONSTANTLY.