She is also older than she seems, when she introduces herself for the first time to Umi and Fuu, and the blu haired girl does tell her she wouldn't have given her more than 11 years, which for a moment makes Hikaru pout but then she forgets it fast.
He hair is longish, but still cut in the way to make her tomboyish. It looks like short hair (that actually somehow has a form of fire) and then she has a really long ponytail that she keeps braided with a finishing touch of a small red bow.
Her eyes are also fiery red when you check the artbook art, showing that when it comes to Hikaru the main color is the one of fire: red with adds of orange and yellow.
We are first introduced to Hikaru in her public school for girls uniform, which naturally has red to it. The chest peace is red, while the skirt is black with a red line on in it. She has something similar to the classical sailor uniform lot of schools have, so while the shirt is red, the bow in front of it and the shoulders are white, with a red line on the bow. (as you can see from the image)
The first armor Hikaru wears - the one that Guru Clef gives them at the beginning, is and Evolving armor, which mutates the moment they get stronger.
The first version consist of a chest pieces, a shoulder protector, a wrist protector on one hand and a half glove on the other hand (I have no idea, because on some drawings is wrist protector on left, and then on other is on the right), along with boots that arrive to her knees. It's mostly white, but there are ornaments in yellow, while the crystal spheres are in red.
First evolution happens, after they pass the test in the Eterna spring to get the Excudo. This time the armor becomes a little bigger, it covers completely her chest and now there are two shoulder protectors. The armor still is more rounded on the shoulders, and still the colors is white, with golden ornaments and red spheres.
Second evolution of the armor happens after Umi is accepted by Selece, the armor starts begining more complicated, the shoulder protectors are now made of two parts, and they don't protect just the shoulders but also the upper arms. The chest peace is composed by at least three pieces which consist of neck protector, and two chest protectors, ornaments are still in golden so are the red crystals, and the armor is still predominantly white colored.
Third evolution of the armor it's done after the Air Temple and Fuu getting her Mashin. This time the change is quite more visible. First thing you notice is that from white it became red. It slowly also gives the idea how the armor will look at the end. The shoulder protectors aren't rounded anymore, but are more elongated and the whole armor is more elaborated. This time around it has a belt which is white, the same way as the wrist protector and the glove. While the red is predominant color, golden decorations are still part of it.
The last armor evolution happens after Hikaru gets her own Mashin, and this time, the armor becomes completely elaborated, including a tiara, that is trying to show the form of fire, and it's colors are red, white and golden. The shoulder protectors are now elongated and their border is with yellow ornament, the chest transformed in a whole piece which looks like a miniskirt, all combination of red (which is the predominant color), white, black and golden. The writs protector and the glove become both white gloves, but they are a little longer and arrive to the half of lower arm, and the white boots are still under the knee, and just a golden with red crystal knee protector covers them.
When they get back in the second season, clef gives them again an armor, which a little more elaborated the first version of the armor but they change it back to the last evolution the moment they use their mashins. Anyway the Second part armor is composed with the combined shoulder piece which is a little elongated, a chest piece, wrist protector, glove and boots to the knees. Predominant color is again white with golden lines and red crystal balls.