Does Kylo Ren's "Resilience" ability refer to an enemy or an ally?

The card text of Kylo Ren's Resilience abilit reads as follow:

Whenever Kylo Ren is in close proximity of another villain he is strengthened and deals more damage with his lightsaber strikes.

But as we know from Heroes vs. Villain playmodem, the light side heroes are refered to as heroes and the dark side heroes as villains.

So does the boost of his card trigger, when he is close to an enemy hero? Or does it mean it triggers when he is close to an allied villain?

While this would be best verified by someone who can test the mechanic, the phrase "another villain" implies that it will only trigger when he is next to another character from the collection of characters classified as villains (of which he is one).

If this mechanic referred to whether or not they were on your side then the "another" part couldn't apply as you wouldn't be able to be against yourself. It probably also wouldn't use the villain keyword, instead opting for ally or enemy like in your title.

It triggers when he is close to an allied villain.

Villain is just the classification/name for "dark side hero" in Battlefront 2, an enemy hero is not considered a "Villain" when you are playing dark side.