I've been thinking about this for years now, but with my current playthrough of Final Fantasy IX on Steam, I felt like it was time to address this.

The following is from a scene of Disc 2 of IX.

[The scene changes, resuming inside the moving cable car. Dagger walks and sits across from Marcus]

Dagger
"I think we're almost there."

Marcus
"So, you already heard about the attack on Burmecia."

Dagger
"Of course. I'm not like Steiner."

Marcus
"You're changed."

Dagger
"Me? You mean the way I talk?"

Marcus
"Not just that."

Dagger
"Well... A lot has happened."
"Oh, speaking of which...!"

Marcus
"Yeah?"

Dagger
"I've been through tons of battles. I'm a seasoned pro, now!"

Marcus
"What are you talking about?"

Dagger
"We need to find Supersoft and save Blank, right?"

Marcus
"You're gonna come along, even if I refuse, right?"

Dagger
"Of course I am! This is so exciting!"
"I've always wanted to see the marvelous architecture of Treno. I can hardly wait!"

Marcus
(On second thought, she hasn't changed that much.)

[Screen fades to black. The cable car arrives at its destination, and Dagger and company exit it]
How would Marcus know how much Garnet has changed? He didn't really spend much time with her. He was just on stage with her briefly during the play and after that, she was kidnapped in the forest and Blank went to help not Marcus and not to mention she was unconscious anyway. After that he never saw her again. How would he have an opinion on Garnet's personality when he really never spoke to her?