I was the one who said that, it was in the season 2 predictions thread.
anyway, if I were to take one guess, I would say he might be some form of an adult... he would be able to control everything electric still.Maybe a cape. Mainly in black. Just a guess though. I still think that the first time he will be seen is in Lyoko. The way he would do it is he probably makes himself some form of ai. SOmething like Aelita. and then, he would use some form of coding to merge himself with the ai. Then he would use the coding from Jeremies program that he used to materialize Aelita to materialize himself. that is possible, Xana is part of the system, and there is nothing showing that Jeremy took out the disk he used for the program, meaning Xana could veiw the code. That is how he would get out of Lyoko. But before he can do it, a member of the gang (I would guess ulrick) is in Lyoko helping Aelita. When he sees the ai. (Now merged with Xana) this would be the second time you heard Xana's voice. Then they would fight. Ulrich would lose all of his lifepoints, get materialized. And then Xana will materialize himself, and then they will spend two or three episodes trying to get him back in Lyoko, or the rest of the season trying to destroy him. in the third season, they might be looking for the creator of Xana, and Lyoko.
An extra twist would be the fact that if they killed Xana, Aelita would die too.Meaning Jeremy would have to get rid of the Virus in Aelita.
Also, I believe this would happen within the last 5 or 6 episodes.