Waddle Dee is a recurring character in the Kirby series. There are thousands of Waddle Dee's, each who serve King Dedede. They are generally harmless, as all they really do is walk (like Goomba), but some have weapons, like a Parasol, Spear, and more. They are replaced by Yarn Waddle Dee's in Kirby's Epic Yarn. In Super Smash Bros. Battle Royale, they serve as multiple things, namely an assist and a projectile for King Dedede.
As an Assist
Waddle Dee isn't a very powerful assist. Waddle Dee is less of an attacking assist and more of an annoying assist- He can bother your opponent very well. He's one of the very few assists that works well as an edgeguarder, as well.
Anti Air: Waddle Dee performs a quick jump upward. As an Anti-Air this move is lackluster, only deals about 6% and it has low priority. However, it acts as a recovery; something assists lack. This makes Waddle Dee's edgeguarding game even better. Overall, it's okay.
Grab: Waddle Dee takes out a parasol and lunges forward. If Waddle Dee hits the opponent, they are shoved in the direction Waddle Dee is facing. It's a Grab in the sense that it hurls your opponent in a certain direction, but... It's a very good move for one reason. That reason being that your opponent is lunged so far away. You can get quite a few really good KOs with this move, even though it deals no damage. It also helps when you're trying to recover.
Projectile: Waddle Dee takes out a bow and arrow, and fires an arrow an the opponent. It's all right, just slow and weak. It does absolutely nothing to damage scaling, so you can use this to link small combo's into big ones... again, if it hits. Deals 4%.
Powerful Attack: Waddle Dee takes out some stone blocks from Megaton Punch. He places them forward and punches through them. By mashing buttons you can make Dee punch through the blocks stronger, covering more distance and damage. Deals 10% at minimum and 50% at max. Good damage on Dee but hard to land.
Waddle Dee Works Well With...
Anyone who can deal damage. People like Bowser and Captain Falcon are very good people for Waddle Dee. Characters with a poor air game like Ganondorf and Link also benefit from Dee. Poor recovery characters like Link and Lloyd Irving work pretty well with Dee.
Waddle Dee Works Bad With...
Perhaps fittingly, Waddle Dee works horribly with Kirby. Generally, any character that has a great air game already and has no real need for someone to help them in the air should ignore Waddle Dee. Tron Bonne also works poorly with Dee. Yoshi isn't too bad with Dee but there are better assists for him.
Poor damage dealers should stay away from Dee, as well. Zero is the type of character that should ignore Dee, although others should watch out as well.