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Gaap wears the typical school uniform at Babylus except that he doesn't have the complete bow tie (having only the button-like demon eye ornament), nor the side pockets. He also has a belt-like strip of cloth along his waist where he keeps his sword. His collar is folded much like Purson Soi's and his coat is longer than the standard, almost reaching his knees. His hair is silver and looks very close to Kerori's natural hair color. His skin also seems to be of the same color.
Gaap can be described as someone who seemingly helps others without gaining anything in return. He's very persistent and has difficulties in comprehending others' personal space. In reality, he helps others because of his ambition "to make a hundred allies".
Not much is known about Gaap prior to attending Babyls, however, It is implied that his friendship with Agares was established prior to the main storyline.
Entrance Exam Arc
During their first activity, which was the flying race, Gaap was given the rank of Aleph (1) based from his performance from the activity.
Cannonball Execution Arc
Gaap was assigned to Team B along with Kerori, Caim, Purson, Allocer, and Alice. He was eventually taken out of the game, and their team eventually lost.
Walter Park Arc
After their finals examination, and being promoted to Beth (2), due to his above-average test scores, Gaap went to Walter Park along with the rest of the Abnormal Class. During their visit to the amusement part, he was grouped with Shax, Jazz, and Caim, with Kalego as their entourage. While they were wandering in the park and trying to have fun, the group Six Fingers suddenly attacked by summoning three beasts in order to create a diversion while breaking out Amy Kirio from the jail below.
During the attack, Gaap along with his classmates asked Kalego to help them take out the beast (Taurus Mountain Bull), but to their dismay, Kalego told them that the battle would be a great opportunity for them to hone their skills and therefore refused to help them. Under the leadership of Jazz, they eventually take down the beast.
Harvest Festival Arc
During the Harvest Festival, Gaap teamed up with Agares and set up a castle within the forest. He actively helped other students in order to stay in the harvest festival due to his ambition which is "to make 100 allies". During the four-day festival their group eventually clashed with Crocell and Caim's Beast Empire for points, leading to an eventual stalemate.
Due to their efforts and unique strategy of using a castle as a defensive strategy leading to significantly fewer students dropping out of the Harvest Festival, both Gaap and Agares were promoted to Gimel (3).
Music Festival Arc
Gaap was part of the Abnormal Class' musical entitled "Lilith's Red Carpet" where he was part of the dance group, and also portrayed the Kind Warrior. After the Music Festival the entire class was promoted to Daleth (4) for their performance (except for Iruma who was promoted to rank 5).
Kamaitachi (Wind Sword) (風太刀（かまいたち）): Gaap's inherited magic from the Goemon family. He can create invisible blades from the wind with extremely flexible movements. These blades could also be thrown similar to a boomerang.
Wind Blade: Kill: Gaap gathers wind until it became concrete and connects with his sword and quickly lashes it out to the opponent or an item and cut it in half. These winds whirl around the sword and maintain a light blue color until the sword is sheathed back.
Rescue Gale: An enhanced version of Wind Blade Gaap learned while training for the Harvest Festival. Allows him to strike opponents over a wide area, but leaves allies unharmed.
It's revealed in chapter 111 that he befriended Agares even before they were paired up for the Harvest Festival. The two seem to be good friends, though Agares was reluctant at first.
He was assigned alongside Agares to Lady Vepar, whose plan is to enhance their bloodline abilities. She emphasizes that their abilities are strong, but only on the ground, so she trains the two to overcome situations where they cannot use their abilities by trapping them underwater.
In demonology, Gaap (also spelled as Goap and Tap) is a Prince of either the southern or the western zone of Hell (President or King in some versions). He can teach philosophy and all liberal sciences, cause love or hate, make men insensible and invisible, deliver familiars out of the custody of other magic-users, give true answers concerning the past, present, and future, carry and re-carry men and things from one nation to another at the summoner's will, and can make men ignorant. He also has control over the Water Elementals or 'water demons'. He is said to be better conjured to appear when the Sun's in a southern zodiacal sign, and should be honored with sacrifices and burning offerings. He is depicted in a human-like shape.
In one of his early chapter appearances, his apparent 'fur' blows in the wind and reveals what seems to be a humanoid chin.