Bookends: Sonic Adventure begins and ends with Sonic fighting him, in their normal and super forms, respectively.

At which point, the only thing you can do is get your affairs in order.

Beware the Quiet Ones: Chaos never speaks and is perfectly content to be left alone.

Beware My Stinger Tail: Chaos 6 uses its tail as only one way of attacking.

His Perfect form grows to be higher than a skyscraper.

Attack of the 50-Foot Whatever: Chaos 0 is barely taller than Sonic.

Attack Its Weak Point: All of his forms share one singular weakness: His exposed brain, the only part of his body tangible enough to attack.

The advances in technology have really done wonders for his watery look, as seen in his appearance ◊ in Sonic Forces.

According to Word of God, this was how he was supposed to look like in Adventure, but the limitations of the Dreamcast was what forced Sonic Team to go with the less detailed model in that game, which presumably also leaked into the FMV cutscenes with them having to use that design instead.

Perfect Chaos gets somewhat of a redesign in Sonic Generations, gaining a chitinous, reptilian scale-like armor on his back, a glowing orange brain and greenish energy emanating inside it.

Most likely he could've mutated from a Neutral Chaos Chao, as they resemble a Ridiculously Cute Critter version of his base form.

All There in the Manual: Sonic Channel says he's a Chao mutated by Chaos Energy.

Was the Final Boss of Sonic Adventure, and makes occasional other cameos. Robotnik eventually set him free, but Sonic was able to calm him. A water being that originally acted as a guardian of Chao but went on a rampage when they were attacked by Pachacamac's clan and was sealed inside the Master Emerald by Tikal.

Shadow in sonic 1