Feel the Magic: XY/XX

Successfully complete story mode on the normal difficulty setting to unlock Body 2 and 3.

Look on the top screen to see which lane the person is flying down. Then, pay close attention and firmly stroke a straight line down the path the man in the cart will soon come through. Every spike-ball will move just enough for the cart to get by. This level is otherwise nearly impossible to complete.

Have Sonic Advance 3 inserted into the GBA slot.

Beat hell mode.

Complete the indicated tasks to unlock the corresponding costume.

Angel's costume and wings: Find 182 stars and 60 rabbits.
Blue and white dress: Find 75 stars.
Blue and white schoolgirl uniform: Find 20 stars.
Blue chicken outfit: Find 100 stars and 35 rabbits.
Blue polo shirt and white skirt: Find 15 stars.
Cheerleader uniform: Find 10 stars.
Cowgirl outfit: Complete hard mode.
Green and white armor: Find 170 stars and 55 rabbits.
Green schoolgirl outfit: Find 182 stars.
Green top and brown shorts: Find 30 stars.
Gray and white dress: Find 100 stars.
Nude: Complete Hell mode.
Orange and white dress: Find 130 stars.
Pink dress: Complete Normal mode.
Pink top and bunny tail shorts: Find 50 stars.
Police uniform: Find 130 stars and 40 rabbits.
Red and white Christmas outfit: Complete Hard mode.
Red swimming costume: Find 170 stars.
White bikini and denim hotpants: Complete Normal mode.
White nurse uniform: Find 100 stars and 30 rabbits.

Go to the right-hand or left-hand corner of the level and remain there patiently. Eventually a vine will come down inches away from you, but not quite hitting you. Just burn it and go back to the corner. Needless to say, you should be as close to it as possible. The vines cannot hit you if you are tucked in a corner well enough. Eventually you will begin throwing your stakes of fire into the creature's mouth. And as for this part; do them one at a time if needed. It is better than losing one third of your health just from hitting his teeth once.

When you go to play Nighmare, in the bull section when the short cutscene play you can see the girl will bull horns and a bull tail.

Simply set the language to Japanese, and the girl mysteriously loses her top.

Complete the indicated tasks to unlock the corresponding hairstyle.

Chao wig: Complete Hell mode.
Lilac ponytail: Find 1 rabbit.
Long blonde hair with blue ribbon: Find 19 rabbits.
Long blonde hair with police Hat: Find 130 stars and 40 rabbits.
Long blue hair: Find 49 rabbits.
Long blue hair with cowgirl hat: Complete Hard mode.
Long brown hair with nurse's hat: Find 100 stars and 30 rabbits.
Long brown hair with white visor: Find 6 rabbits.
Long gray hair with chicken hat: Find 100 stars and 35 rabbits.
Long gray hair with red and white hat: Find 29 rabbits.
Long gray hair with Santa hat: Complete Hard mode.
Long gray hair with white bunny ears: Find 59 rabbits.
Long orange hair: Find 5 rabbits.
Long pink hair with halo: Find 182 stars and 60 rabbits.
Long purple hair with white cap: Find 39 rabbits.
NiGHTS wig: Complete Hell mode.
Puyo Wig: Complete Hell mode.
Short green hair: Find 3 rabbits.
Sonic wig: Complete Hell mode.
Ulala wig: Complete Hell mode.

If its January 1st (or if you change the system's clock), then when you go to the main menu screen, the girl will wish you a Happy New Year.