"'Cause I... I love you... I want you to know... that I... yes, I love you... I'll never let you go." ~ 10th Avenue North
"You may be my master, but I will NOT let you hurt my friend!"
Eraqus was like a father to Terra, but that didn't mean that he would just bow to his every whim. Ven was almost literally the little brother he'd never had, and he wasn't going to let him get hurt if he could help it. Family stuck out for each other. And he'd stick out for Ven no matter what.
Had it been Aqua, he'd have done the same. Terra would do anything for his friends. He loved them both, albeit slightly differently, and he would always protect them, no matter what his master said. He just hoped that the man could understand that.
With that in mind, Terra brandished his keyblade, hoping that he'd be able to knock a little sense into his teacher by the end of this tussle.
Oww... right in the feels. We all know what happens after this battle. Fourth saddest scene in the whole game (after Xion, Roxas and Axel's deaths, in that order). :'(