The Watcher, called "Surveillance Robot" in Kingdom Hearts II, is an Emblem Heartless introduced in Kingdom Hearts II.
Kingdom Hearts II
A Watcher is a bronze-colored, funnel-shaped, mechanical Heartless with a diamond-shaped opening in its front, exposing its glowing, yellow eyes. Two black, rectangular guns are mounted on either side of its body. Each of these guns sports a silver, pyramidal spike on both of its ends. There is a silver ring that acts somewhat like a gyroscope hovering around its base, despite not being directly connected to the Heartless. This ring bears a vague resemblance to the top half of the Nobody symbol. It has a silver spike on the top of its head, and its Heartless emblem is just below the opening for its eyes.
The name "Surveillance Robot" obviously refers to its mechanical nature. "Surveillance" may refer to military surveillance, a systematic set of observations, or simply "surveil", which means "to continuously monitor". The Heartless's ability to levitate would presumably make it better suited for surveillance than other Heartless. "Watcher" holds a similar meaning, and can also mean "guard".
Kingdom Hearts II
The Surveillance Robot is not a very difficult opponent, and can prove very useful with its Sparkle Ray Reaction Command. Sparkle Ray won't damage it, however, so Sora will have to attack it directly However, this is especially helpful in the Battle of the 1000 Heartless. Since Sparkle Ray is very powerful and it doesn't damage the Surveillance Robot, Sora can literally use the same Surveillance Robot again and again until all the Heartless are destroyed.
Kingdom Hearts 358/2 Days
In Kingdom Hearts 358/2 Days, the Watcher is more annoying in the sense that it shoots lasers randomly and may shoe-glue Roxas, preventing him from jumping. If shoe-glued, consider using homing magic such as Fire, Aero, or Blizzard, all of which the Watcher is weak to. It is possible to deflect the lasers back at it, but it will counter with a powerful laser that can't be blocked. Either way, it is still a very easy opponent.