The Boss of Dark 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 Dark Shadow 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 Dark 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 Dark 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 Dark 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 Dark 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 Dark Shadow is the final boss and main antagonist of Rainbow Islands. It is fought in the last world, Bubble Island. As Bubby and Bobby enter the room, the Boss awakens from its slumber and moves around the room, shooting bubbles at Bubby and Bobby. After being hit enough times, Dark Shadow's Bubble Dragon form's skeleton crumbles, and he transforms into the Super Skel-Monsta, his true form.
After being defeated, the Super Skel-Monsta taunts Bubby and Bobby, and the room fills with his blood. The two must jump on platforms to escape the blood, which will make them lose all their lives if it touches them.
In the game's ending, he is referred to simply as the Boss of Shadow.
Although the Super Skel-Monsta perished in Rainbow Islands, the Boss of Dark 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 Dark Shadow appears only in its Bubble Dragon form in this game, the Super Skel-Monsta does not appear.
A new incarnation of the Boss of Dark Shadow appears in Bubble Memories in a reincarnated form known as the Super Dark Great Dragon. Though the two are referred to by the same nickname in the Japanese version of the game, but are considered separate characters, despite being incarnations of the same being.
The Boss of Dark Shadow returns in Rainbow Islands Revolution, once again acting as the final boss. Dark 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.
- Upon defeating the Super Skel-Monsta in Rainbow Islands, he exclaims "What strength!! But don't forget there are many monsters like me all over the world". This quote appears to be 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.