Steve?

Character
Steve? is the default male protagonist in Minecraft. There's not much personality to this character as he is only a default skin, but he is commonly referred to as the mascot of the game, spawned many pieces of merchandise and inspired the creation of Alex, the female default skin.

Importance
Minecraft is one of the most sold video games of all times, likely due to it's accessibility, being released on all major platforms with sales numbers of over 140 million in total.

Affiliation with Nintendo
Minecraft has been completely endorsed by Nintendo. Being available on the Nintendo 3DS, Wii U and Nintendo Switch. Microsoft collaborated with Nintendo to release an official Super Mario mod pack.

In addition, Masahiro Sakurai has stated he enjoys playing Minecraft.

Symbol
Steve?'s series symbol is the face of a Creeper, the incredibly iconic enemy from the series.

Palette Swaps
Half of Steve?'s color changes turn him into Alex, the default female skin. All of the skin's colors are based off of dyes from Minecraft that are used to color wool.
 * Steve?: Blue (Default), Yellow, Red & Black
 * Alex: Green (Default), Pink, Purple & White

Kirby Transformation
Kirby's head turns into a pixelated block in the same vein as Minecraft.

Normal Moves
Steve?'s normal moves consist of him using Diamond tools primarily the sword, but also the pickaxe, shovel, axe and hoe.

Neutral Special
Flaming Arrow

Using a bow and arrow, Steve? shoots a flaming arrow, similarly to Young Link's neutral special.

Side Special
Fishing Rod

Steve? uses a fishing rod to grab enemies. Once grabbed, he can throw them backwards or forwards. Can also be used to grab ledges. This functions identically to Isabelle's side special.

I predicted this attack over half a year before Isabelle's reveal.

Up Special
Ender Pearl

Throw an ender pearl in any direction to teleport to the location it hits. Does not stop Steve's descent if off-stage, so time it right. If it hits an enemy, you'll teleport to them. Can be used infinitely, like Yoshi's Up Special.

Down Special
TNT

Steve drops a block of TNT that has a timed explosion. This can be a great ledge trap if you time it properly. The TNT can be picked up after placing and can be thrown, attacking it will not blow it up, but will send it flying, use this to your advantage for some sick timed kills.

Final Smash
Nether Ghast

* Edited to match Ultimate's new FS rules.

Similar to Simon and Richter's final smashes, Steve would summon a portal, specifically a Nether Portal, where the trapped enemies would be transported to the Nether, where a Ghast would shoot fireballs at them and cause massive knockback.

Victory Theme
Steve?'s victory theme is a segment of the song Cat from Minecraft

Trivia

 * Steve?'s original Final Smash had the player turning into an Ender Dragon, giving them abilities similar to those of Yoshi and Charizard's Final Smashes.