Sonic the Hedgehog 4: Episode III

So the Classic Sonic series (or the Origins series) every game has new characters, like:

Sonic 1:Sonic

Sonic CD:Metal Sonic & Amy

Sonic 2:Tails & Mecha Sonic mk.I

Sonic 3:Knuckles

Sonic & Knuckles:Mecha Sonic mk.II

We know Sonic 4 introduced Metal Sonic in Episode II, but Metal already appeared in CD. So what character do you think can be a good fit if Sonic 4 had a new character? (Modern Characters only)

The story (For mines) is that Sonic and Tails crash land on a new island, and go to a ship to go to Little Planet to free it again.

First Level

A Jungle-Death Egg inspired level with Wing Fortress enemies, the main boss is when some sort of orb is magnetic and uses scrap metal pieces to attack. Knuckles tricks Sonic and throws a bomb that leads to the:

Second Level

A water-cave act that acts as a "scary level" with ghosts from Sandopolis, Second act involves a mountain level. Third act being unknown, the boss is a drill.

Third Level

A Sanctuary/Hidden Palace like level, the first act contains a Knuckles fight, who actually is... MECHA SONIC?! Anywho, Mecha's fight consists of homing attacks, a mole attack where he goes under the ground, and goes up and tries to crush Sonic, it is also a race, and he stops in some parts of the level to fight. The second act is actually:

Fourth Level

The Sanctuary part, nothing to say except that Knuckles, the real one, tries to stop Mecha, but gets knocked out, Mecha uses a Chaos Emerald that he had, to continue the fight. He loses, also causes a slight retcon to one of the IDW comics.

Fifth Level

Little Planet, still trapped inside, you go through a Metallic Madness-looking zone for the first act, the second act is the penultimate act that Robot... err Eggman, finally shows up to, there are three phases to this boss that I might post a different time.

Thats all I have.