DESCRIPTION:
No monster boy, no ghost story shall keep me from pretty fireworks!

Shokubai Phantom Girl actually posted profiles for the characters on Pixiv alongside this song. So that's nice!

Name: Kayo Minase [The Girl]
Age: 11
Birthday: October 10th
Height: 145 cm
Weight: 34 kg
Blood Type: O
...A girl in sixth grade. Rather quiet and kind of a crybaby, but a very good runner. She likes dogs, and dogs seem to like her too, so she often brings lost ones home. She's very bad with scary things, and had a traumatic experience with konnyaku. Her favorite food is apple pie.

Name: Tora Amamiya [The Boy]
Age: 11
Birthday: November 11th
Height: 147 cm
Weight: 37 kg
Blood Type: O
...A sports-loving boy in sixth grade. He's handy with his fingers and good at art. He has a brother three years older and a brother four years younger, and sometimes feels like he has to babysit Kayo the same way he does the latter. He's usually relaxed, but when it comes to pudding, the look in his eyes changes and he'll challenge people to rock-paper-scissors for it. He dislikes sour foods and people talking too fast.

Illusory Contours of the Forty-Ninth Day [The Giant Doofy Ghosts]
A result of Kayo's memories from the day the boy was lost to her. Afterimages of a sight she dares not touch upon, tried to forget, and can't look at directly. The contours creep toward her as if to crush her to death. They were memories that could not be forgotten. As a result of Kayo accepting the gruesome memories of her past, they were forced into qualia at their objective center point.

Sparkle Wonderfire [The Calcifer Flame]
A ***** born from Kayo Minase's vivid vision of that day, taking the form of a bright blue and white flame. "*****" are "manifestations of qualia," and thus neutral entities between living and material. The origin of ***** has inherently unique qualities and is a rare phenomenon. Normal ***** have only one visual organ, but Sparkle Wonderfire has two, indicating it may be a mutated *****. Additionally, ***** has a ***** similar to a human mouth in a similar position, which is referred to as an "absorber." This absorber also bears resembles to the ***** on the simultaneously-discovered *****.
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While they look kinda similar, Kayo is NOT the girl from Monster Boy. (The latter has goggles and doesn't have that bit of hair tied up.) Shokubai Phantom Girl has remarked that the two from Monster Boy don't have names yet. And if you'll notice, toward the end of the video...

The original description says it's a story of "the 49th day after death." This is when Buddhist memorial services are held, as it's thought that after seven weeks, a spirit will have been judged and can go on to the afterlife. Hence "Wander 77."

Original video: http://www.nicovideo.jp/watch/sm18640403
Lyrics: http://vgperson.com/lyrics.php?song=rainbowapartments