Rika is definitely different in appearance in comparison to all the other females in the Digimon series. Here I'm going to address the parts of her appearance that make her stand out to me, or rather the things that people most recognize her for.

HAIR: Rika's hair is probably the most noticeable thing when you first look at her. None of the females in Digimon wear their hair up as their normal appearence, especially in a spiky ponytail like her. Rika's hair color, although Sora from Adventure had a similar color, really stands out due to it being up and the highlights because it sort of gives it the appearance of flames. Redheads tend to stand out in a crowd, and Rika really is not an acception because she is easy to spot amongst the other Digidestined.

SHIRT: Rika's trademark item is definitely the broken heart on her shirt (though it does become whole later). It really makes her stand out from the other characters, especially previous females because nothing like that had been done before in character designs. The heart is also symbolic, as it does represent Rika's character development throughout the series. As the ice around her heart melts, the heart becomes a full one that is not broken.

BELT/CARD HOLDER: One last trademark item of Rika's, her belt has been part of her outfit since episode one. Not so much the belt itself, but the card holder in the back. Not only do I think that is really cool, but Rika is the only tamer that has a special place for her modify cards. She is very well known for reaching behind her and pulling out some card to enhance Renamon's abilities, which makes this item a definite Rika thing.