"Yes, you are pretty."
Translation:응, 너는 예뻐.
You can use 너는 or just 너 but the meanings are slightly different. If you say '응, 너는 예뻐', it can mean 'yes, you are pretty (but maybe not them)' If you say '응, 너 예뻐', it can mean 'yes, you ARE pretty (so you should know how beautiful you are! / so stop talking about it)' The reason I said it CAN is because you must consider the context. It varies by the context a lot. I also thought about 너도 and 너가, but then the translation would be different. 너도 예뻐 is 'you're pretty too'. 너가 예뻐 would be 'YOU are the one who's pretty' but it's kind of unnatural. So I think 응, 너 예뻐 or 응, 너는 예뻐 will be the most suitable and natural ones
If this is casual, the most you'd say is "너" while dropping the suffix. However, dropping it altogether is also completely fine and normal. Asiatic languages are hard to teach in a very programmatic way, so this is something you can only really learn by actually speaking with other people of Asiatic descent