The Boss of Shadow (暗黒大魔王 Ankokudaimaō?, "Dark Demon King") is the final boss of Rainbow Islands, and the true antagonist behind the events of Bubble Bobble. The Boss of Shadow initially takes the form of a giant armored Bubble Dragon, while its true form is a giant Skel-Monsta known as the Super Skel-Monsta.
In his first form, the Boss of Shadow resembles a giant Bubble Dragon, specifically resembling Bub. He wears a suit of armor with spiked pauldrons and gauntlets with red stones embedded in them. The Boss of Shadow has two fangs, as opposed to the usual single tooth usually seen on Bubble Dragons, as well as an angry facial expression and small green wings.
In his second form, the Boss of Shadow resembles a colossal Skel-Monsta.
In his first form, which is simply known as Bubble Dragon (バブルドラゴン Baburu Doragon?), the Boss of Shadow's main attack is firing bubbles out at the player to attempt to trap them inside. He is also able to change humans into Bubble Dragons, as seen by the prisoners kept in his lair who have been transformed, much like Bub in the original Bubble Bobble. His second form, the Super Skel-Monsta, is shown to be able to conjure small winged Skel-Monstas to attack its foes in the Game Boy Color version of Rainbow Islands.
The Boss of Shadow is the final boss and main antagonist of Rainbow Islands. He is fought in the last world, Bubble Island. As Bubby and Bobby enter the room, the Boss awakens from his slumber and moves around the room, shooting bubbles at Bubby and Bobby. After being hit enough times, the Boss of Shadow's Bubble Dragon form's skeleton crumbles, and he transforms into the Super Skel-Monsta, his true form.
After being defeated, the room fills with the Super Skel-Monsta's blood. The two must jump on platforms to escape the blood, which will make them lose all their lives if it touches them.
The English manual of the Nintendo Entertainment System port of Rainbow Islands refers to the villain of the game as being named Krabo, though this is never referenced in-game.
Although the Super Skel-Monsta perished in Rainbow Islands, the Boss of Shadow returns as the boss of Rainbow World. It behaves similar to his first appearance, although Bubby must defeat it using Water Bubbles this time. The Boss of Shadow appears only in his Bubble Dragon form in this game, the Super Skel-Monsta does not appear.
A new incarnation of the Boss of Shadow appears in Bubble Memories in a reincarnated form known as the Super Dark Great Dragon. The two are referred to by the same nickname in the Japanese version of the game, though this is not present in the English release.
The Boss of Shadow returns in Rainbow Islands Revolution, once again acting as the final boss. The Boss of Shadow acts extremely similar to its first appearance, although he is not fought the same manner. Bubby, Bobby, Mike, and Violet must draw rainbows using the Nintendo DS touch screen to attack him.
- If the player defeats the Super Skel-Monsta in one hit in Rainbow Islands by collecting all the items in each Secret Room in, he exclaims "What strength!! But don't forget there are many monsters like me all over the world". This quote is a reference to the victory text in the original Street Fighter, which was released just barely over a month before Rainbow Islands, in which the player's opponent states "What strength!! But don't forget there are guys like you all over the world" after being defeated.