Rouge

From Sonic - NeoWiki

It is discovered by Shadow that she is working as a double agent. However, she also has her own intentions - to use Eggman and Shadow's help to gather the Chaos Emeralds for herself.

Following this, she goes back to treasure hunting and attempts to raid Eggman's treasure storage facility. Although she finds no jewels, she does find something else - Shadow. After reawakening him, she accidentally releases E-123 Omega, sparking a fight which she breaks apart. After helping to fight against Metal Sonic, she then takes Shadow to her base. She is soon called on to investigate the robot Emerl. This sparks a significant rivalry between herself and Amy, with Rouge briefly getting the robot. It soon helps Knuckles.

In Sonic X, she was caught in a trap intended to catch Sonic. G.U.N. planted a fake Chaos Emerald to lure him. However, Rouge got there first and was promptly arrested. It was offered to drop the charges if she agreed to spy for them. She agreed, knowing this would grant her unprecedented access to the Chaos Emeralds.