From the Super Mario Wiki
Topmen are enemies from a group called the Topman Tribe that first appear in Super Mario Galaxy. They resemble Tops and are always spinning, trying to attack Mario. They appear in the Battlerock Galaxy, Buoy Base Galaxy, and the Dreadnought Galaxy. They travel throughout space in flying saucers and are led by Topmaniac. The Topmen have also been seen in a Mario Kart Wii competition in Wi-Fi. They are very weak to electricity, as spinning them into electric currents instantly defeats them (with the exception of Topmaniac).
There are three types of topmans. One are little yellow ones that reflect the player off if you touch one. They can be defeated by Spin Attacking them. There are green ones that are bigger than yellow ones that the player can jump off like a spring. The third one is called Spiky Topman. The player cannot jump on them due to spikes on their heads.
Topmen act similarly to the Bully from Super Mario 64. They act in the same manner and have the same powers, both not able to hurt Mario with their normal body slams, although Topmen can damage Mario if he touches the spikes on their heads. Like Bullies, they push Mario into something more dangerous, like freezing water, electricity or lava.
Super Mario Galaxy Trading Card Description
Under the rule of Topmaniac, Topmen are part of the Topman tribe and are always trying to attack Mario by spinning into him. Mario must avoid these toplike enemies in the Battlerock Galaxy and the Dreadnought Galaxy.
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