XANA's Kiss Code Lyoko : Season 2 Episode 17

Xana creates a ghost called a polymorphic specter, that can change its shape or form at will and doesn't need to possess a host, then uses it to drive a wedge between the group by having it kiss people while disguised as them. At the end of the episode Aelita kisses Jeremy to see how he'll react. He (predictably) freezes up.