My uncle works at Nintendo so I got this straight from the source. An upcoming DLC will add unique T shirts for each character! Only one image has been done so far but I have the full list in text form.
Byleth(male): "No you're right, let's do it the dumbest way possible because it's easier for you"
Byleth(female): "Is whatever this is almost over?"
Claude: "Sarcastic comment loading please wait"
Hilda: "I get enough exercise just pushing my luck"
Lysithea: "Boy do I hate being right all the time"
Raphael: "Destroyer of pizza buffets"
Ignatz: "Nerd"
Lorenz: "Very stable genius"
Leonie: "Beast mode on"
Marianne: "Can I pet your dog?"
Dimitri: "What the fresh hell is this?"
Dedue: "I'm not shy I just don't like you"
Sylvain: "Keep laughing this is your girlfriend's shirt"
Felix: "All damn day"
Ashe: "Good vibes only"
Mercedes: "Close enough to perfect"
Ingrid: "I am destined for success! The cookie told me so"
Annette: "Go shawty it's sherbet day"
Edelgard: "Legal tip: demand a trial by combat"
Hubert: "See you in hell"
Bernadetta: "I want to go home"
Linhardt: "Whatever can go wrong will go wrong"
Dorothea: "Dearly beloved"
Ferdinand: "I really do care, don't you?"
Caspar: "This is pretty much my final form"
Petra: "The night is dark and full of terrors"
Nothing for faculty yet, but I totally expect Alois to have something to do with dad jokes.