For the Caterpillar, I would have IU play that role. IU is a very sweet girl, but she can be very inspirational and funny. I can imagine her saying her lines and it would make the role 10x better. IU is an actor-singer. She has much experience in roles that are inspirational and funny. I think she would be the best choice for this character.
SNSD YoonA - she's probably the most beautiful kpop artist for me. She looks good in most(if not all) angles but more than that she has this bright personality that pulls people. She doesn't try too hard to look pretty and she likes to ruin her image but she still ends up looking good! :)