Sumi Kuroi is an arrogant girl with a fiery personality. She considers Ink her rival.
Sumi looks like a regular grade-school studend like Ink, but is actually in high school.
She has short purple hair with a stray strand that changes depending on her mood.
Sumi is also in love with Nao and becomes Ink's rival. She is 17 years old even though she looks like a grade-schooler. She is very rich and has a spoiled attitude because of that.
She and Ink were friends when they were younger but for unknown reasons they broke apart (probably because of jealousy since she is horrible at English), even though Ink still tries to be Sumi's friend.
A few quirks of hers are that the end of the long strand of hair coming from the front of her head usually portrays what ever she is feeling (!,#, gloved finger pointing); and that she usually falls asleep while studying, which results in her getting hit in the head with a slipper by her maid Ruriko.
By the end of the series it seems that her crush for Nao has all but faded and she has started to develop feelings for her magical partner Ka-kun (he has feelings for her as well) and has become good friends with Ink once again.
Sumi changes into her magical form Tempera Sumi using a red flip phone, through the use of stars, putting her in a cat style outfit with her hair turned red.