Code Lyoko

Four students from Kadic Academy being Jeremie, Ulrich, Yumi, and Odd, discover a supercomputer housing a virtual world called Lyoko. Inside, they meet Aelita, a sentient virtual girl. Together, they must travel to Lyoko to fight off XANA, an evil multi-agent virus attacking Earth and the digital realm. Xana uses the towers in Lyoko to send attacks on earth by infecting machines and Aelita is the key to enter the towers and stop the attacks. You are Aelita childhood friend who volunteered to go into Lyoko with her and her father to protect her, but like Aelita you lost your memories, but still kept your mission to protect her. You appear when Aelita is in danger and attacks anyone near her, your weapon is a Sniper and power is to turn invisible and to change weapons. You are given the name Stalker by Jeremy and them. This takes place on Season 2 episode 2, where Aelita gets attack by the Scyphozoa in sector 5