This titan boss/ally/friend of Kirby in the Kirby series has been in Super Smash Bros. games since Brawl. His incarnation here is pretty strong. The guy who created the Kirby series also created Super Smash Bros. so it's not a shocker that King Dedede isn't weak.
One of his best moves is the gordo throw where he tumbles a spike at you. It hurts a lot, so you better to avoid it. It is easily sent back his way, so King Dedede can be countered rather easily by certain players and characters.
Despite his power and weight, King Dedede has an amazing recover. He floats and flies just like his enemy/pal Kirby. It does take some skill to knock King Dedede to the edge of the screen.
His Final Smash is the Dede-Rush which is an ode to his incantation in Kirby Super Star Ultra where has a metal mask and puts one unlucky player in the wrestling cage. Then they're hit by a mighty hammer. Not many survive this one.
When Kirby takes King Dedede's power he gets a different kind of inhale. He can turn enemies into big stars, but also catch and send back projectiles. It's a decent ability for both warriors.
Online I did okay with King Dedede but I've seen people way better than myself use him with excellent results. I like his moves and power, but he isn't the most agile fighter. But even if I lost a match, I didn't go out easy.