Sol, called Gola (ゴーラ) in the Japanese manual, is a Badnik from the 16-bit version of Sonic the Hedgehog 2 found in Hill Top Zone. It carries four fireballs, which rotate around its body. When the player gets close, it fires them straight ahead. Sol can be defeated by ducking and waiting for it to release all four fireballs - it is then completely defenseless. Sol is similar in design to Orbinaut, a Badnik from Sonic the Hedgehog.
A second Sol, simply known as Gola, makes an appearance in Sonic Blast where it can be found in Red Volcano Zone. Unlike the ones in Sonic 2, these ones only have a single fireball orbiting their bodies, which they prefer defending themselves with rather than firing it at the player. These enemies are hard to avoid while crossing lava pools, but their lack of fireballs make them easier to destroy.
Another Sol, known as Gola (ゴーラ) in the Japanese manual and by what it's mostly called, appears in Sonic Adventure. It's very close in design with the original from Sonic 2. It usually hovers around orbited by the cluster of flaming orbs and it attacks by expanding or contracting said cluster to adjust the position of the attack radius. They are found in the following Action Stages:
Sonic Adventure 2
Yet another Sol, known as Gola (ゴーラ) in the Japanese manual and by what it's mostly called, appears in Sonic Adventure 2. It's very close in design with the one present in Sonic Adventure, but it has four distinct fire balls, and though it has the same behaviour and attacks, once a fire ball hits an enemy, it will disappear. It awards 100 points when destroyed.
Sonic Advance 2
The Badnik's next appearance is in Sonic Advance 2, where they are found in Hot Crater. In this game, their fireballs orbit around the front and back of their bodies much like in the Sonic Adventure titles. They have to be taken out by jumping on top of them with a Spin attack.
Sonic Lost World
Sol makes another appearance in both versions of Sonic Lost World. In the Wii U/PC version, they are found in the 3D sections of Desert Ruins Zone 1 and Frozen Factory Zone 1 where they either remain stationary or move about. When the player approaches them, they will make their fireballs orbit around Sonic and close in on him, which must be avoided by jumping as they close in. They are also found in the 2.5D sections of Sky Road Zone 4, they remain stationary in-between ceilings while lines of fireballs orbit them laterally.
In the Nintendo 3DS version, they are found in Desert Ruins Zone 1-3. In the 3D sections, their four fireballs orbit them horizontally, while in the 2.5D sections, they are found in wide areas or tight cavities, where they have two-to-four fireballs orbit them laterally. The best way to deal with them is to use a Somersault Kick to stun them, then strike them with a Homing Attack.
The Badnik's sixth appearance in the series is in Sonic Mania, where it is found in Oil Ocean Zone Act 2. They use the same design as in Sonic 2, except they have two fireballs orbiting them instead of four. They will hover about, usually above pools of oil or oil slides, protecting themselves with their fireballs. When destroyed, their fireballs will scatter everywhere, igniting the oil should they land in it.