King Dedede is most similar with Hammer Kirby, with less power but more health.

General

• King Dedede does considerably less damage on all his attacks than Hammer Kirby, and Meta Knight is more powerful in every given scenario. Thus, Dedede should never used for combat in co-op runs.
• King Dedede always has a hammer. This makes him handy for energy spheres that require pounding stakes, destroying metal blocks, or firing a projectile (thanks to unlimited Hammer Throws) so Kirby will not have to discard his current ability.
• King Dedede's attack underwater is much faster and less laggier than Hammer Kirby's. However, it doesn't cover all angles, only above and in front.

Character Differences^[1]

• Hammer Flip isn't fire type
• Can't do Giant Swing/Ultra Giant Swing like Hammer Kirby
• Less damage on most attacks than Hammer Kirby
• Dedede only appears larger than the other characters, however this isn't actually the case. Attacks to his head will do no damage.
• 25% more HP than Kirby
• Slightly more range on his attacks than Hammer Kirby
• Can do Hammer Throw as many times as he likes
• Underwater attack is different from Hammer Kirby's

Moveset

Hammer (Press 1)

• Attack properties ^[1]
  • 26 dmg
  • Star hit - 18 dmg

Hammer Twirl (Down + 1)

• Attack properties ^[1]
  • 1st hit – 16 dmg
  • 2nd, 3rd, 4th hit – 14 dmg per hit
  • Max total – 58 dmg
  • Invincible for the duration of the move, can be used to spot dodge attacks or invincible aerials.

Triple Dedede Hammer (Hold 1, release)

• Attack properties ^[1]
  • 1st & 2nd Hammer hit - 16 dmg per hit
  • 3rd, hammer hit - 32 dmg
  • Max total for hammer - 64 dmg
  • 1st, 2nd, 3rd Star hit - 12 dmg each
  • Max total for stars - 36 dmg

Hammer Flip (Up + 1)

• Attack properties ^[1]
  • Hammer hit only - 45 dmg
  • Hammer hit after aura hit - 39 dmg
  • 1st Aura hit - 8 dmg
  • 2nd, 3rd, 4th, Aura hit - 7 dmg per hit
  • Max total - 68 dmg
  • Non-elemental, can't light fuses or destroy ice blocks.
  • Hitting with the back of the hammer has the highest potential for damage compared to hitting with the front, thanks to the aura.

Midair Hammer Swing (1 in air)

• Attack properties ^[1]
  • 26 dmg

Giant Dedede Swing (Dash + 1)

• Attack properties ^[1]
  • 26 dmg
  • Invincible for the duration of the move.

Dedede Hammer Throw (Dash + (-) or Giant Dedede Swing + (-))

• Attack properties ^[1]
  • Spin hit – 26 dmg
  • Thrown hammer - 32 dmg
  • Max total – 58 dmg
  • Dedede has infinite hammer throws
  • Dedede is invincible during the spin, but vulnerable during the actual throw.
  • More momentum and movement range on the ground if performed from a Giant Dedede Swing.

Dedede Team Attack

• To execute, have all players on each other's back with King Dedede on the bottom. Everyone hold guard until the attack is charged, then release simultaneously.
• Attack properties ^[1]
  • 2P - 45 dmg
  • 3P - 57 dmg
  • 4P - 74 dmg
  • King Dedede slams his hammer onto the ground and creates 5 stars that form an arch-like pattern around the team.
  • Shortest range out of the 4 team attacks.
  • Stars linger after the attack then disappear.

References

1. ↑ ^{1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8} An ability and damage guide at the Kirby's Rainbow Resort forums. It's mainly maintained by Plas Durock and contains contributions from several members of the Rainbow Resort forum and the GameFAQs KRtDL forum. http://www.gamefaqs.com/boards/935607-kirbys-return-to-dream-land/62398037