He is similar in appearance to Genie but mildly obese and dark pink and has fangs and sharp teeth.
Unlike Genie, the Evil Genie is a being of pure evil that delights in causing destruction and suffering. Its also more animalistic, acting like a savage monster incapable of speech.
Because he is not freed, he is far more powerful than Genie and has infinite cosmic strength at his control. But because he is still bound by the lamp unlike Genie, he is obligated to obey anyone who holds the lamp.
Chaos creates an Evil Aladdin who summons an Evil Genie, which starts to rampage through Agrabah. Genie tries to fight him, to no success. As Mirage is gloating, the Genie captures her and she reveals his weakness is the lamp so Aladdin destroys the lamp, causing the Genie to disappear and causing Mirage to technically save the day.