Jasper (Sauville Arc)

In Season 2, Jasper was brought in to experiment on the general populace with the explicit goal of transforming them into beings not unlike the Great Beast. Alongside that, however, he performed research on how one can travel from various worlds through the use of 'summoning', and if there was a way to utilize this without means of a contract. Both of these continue for some time undiscovered, but when Jasper realizes how much of a threat Sauville is, he decides to target them by his own means.

This ends up being through deception -- after stealing Nahe away, Jasper claims that he's a close friend of them and requires the mercenary group's assistance. The wary but willing crew heads on down, fights off some of Jasper's creations, only to then be caught and experimented on. Fortunately, however, Nahe is able to contact the rest of the crew waiting back at Sauville through Sully and, after a bit of explanation, another crew is dispatched to save everyone.

While Jasper is put into Sauville's jail (his robotic limbs taken away as punishment), not long after Sauville is saved, he mysteriously disappears...

...and finds himself in a space unknown. 

This was just the start of many oddities that would occur during Season 3, where the multiverse's world borders were beginning to break and collapse. And as Jasper begun to get an understanding of where he was, he realized that this space, if he could call a world, was full of spirits that held an immense power. Such a power that, when able to work with them to such a high degree, would form into whatever he desired -- and Jasper knew at that point, he couldn't leave until he learned all of this space's secrets.  

Eventually, two things happen that break this otherwise endless study. The first is the appearance of Vera -- brought to this world by means similar to how he got here, Jasper quickly uses his powers to control the person once more. The second, however, is the appearance of a different, but just as annoying mercenary group, who by miracle alone had come to this space to save Vera. Both of these are done successfully, and Jasper gets a fair number of injuries doing so...but ultimately, it's unknown just where he's gone off to.