The same weavile as the one from the Daycare Comic. Since Weaviles don't naturally have a mega form, Karen obtained hers from making a little "deal" with Giratina. Though all her natural abilities and strengths are boosted, her form is more geared towards enhancing a particular favorite recreational activity she enjoys partaking. Increasing her vitality and strength the more she "drinks" from other pokemon's excess aura... and if they're not careful, their life force as well. This complements her well in her team given her role as the expert in weakening the enemies' endurance and capturing/detaining pokemon.
The price for this "gift" was the obligation that Karen had to return some of that gathered aura energy to the Reverse Realm to help Giratina rebuild its torn world... a contract that went sour and had a falling out, leaving the weavile and giratina parting on less than friendly terms. Despite this, Karen has managed to learn the renegade's special skill: SHADOW FORCE, and can use it to slip between the shadows, out of sight, and even ambush-strike enemies... albeit not as effectively as Giratina itself could do, but still an effective skill she had stolen nonetheless. Using all these special powers drains the weavile's demonic energy, which eventually must be replenished the good-ol-fashioned succubus way.

It's also a little explanation for Karen's higher than average libido.