Claymore

Claymores are the Radiant Garden's defensive system, installed presumably by Cid, who also presumably gave them their name. They are useful to the Hollow Bastion Restoration Committee because they help defeat Heartless and Nobodies that maybe anywhere in the town. After you leave the Castle Gate area, the Claymores can no longer help you in battle.

Story in Kingdom Hearts II
During the third visit to Hollow Bastion, the MCP has taken control of the Hollow Bastion OS which includes the Heartless Manufactory and changing the Claymore's programming so they attack Sora instead.

Appearance
Claymores are spheres of light that appear from the ground. After forming a circle beneath the target, the sphere is launched up to inflict damage. The damage, unfortunately, is very minor as it can only send the target flying into the air at best which makes it a perfect target for Sora's aerial attacks.

When they begin to attack Sora, they slightly change in appearance and they also make a different sound. They still do not deal a lot of damage and they are pretty easy to dodge.

Trivia

 * In Re:Chain of Memories, in the 13th floor and 1st basement, a system similar to the Claymores is present to attack Sora and Riku.
 * These are not to be confused with the weapon of choice of Saïx.