During the Future Arc, the Tenth Generation Vongola Family gets sent ten years into the future. There, they battle against a force known as the Millefiore Family. Much of their battles are based on their particular elements. In this time, a new type of weapons was invented. Box weapons store items and weapons, these weapons can take on the shapes of actual weapons. In many cases, though, they also take on the form of a unique animal. The catch is that most of the box weapon animals had to have existed at some point in history. Xanxus' box weapon is named Bester (Besta) and is a male (the mane gives it away) liger. Originally mistaken for a Sky Lion and later confused with the Storm Tiger, Xanxus says that Bester is neither a lion nor a tiger. It is in this arc that we learn that the unique properties of the Flames of Wrath are actually a combination of Sky and Storm flames. Bester is the hybrid of these two elements and represents harmony and destruction. Another ability allows Bester to combine with Xanxus' Dying Will Guns which makes them even more powerful.
The box weapon animal is dependent on the flames and the rings of the user. Xanxus wears a sky ring because he is a sky element. His unique flames of wrath have a storm property to them. When combined into the box weapon, the usual sky lion is transformed into a sky and storm hybrid liger. When summoned with just the sky ring, Bester appears as portrayed on the left. When summoned with the flames of wrath and the sky ring, Bester appears as portrayed on the right. Interestingly enough, when Xanxus is pissed and his scars start to show, Bester's stripes show or get more prominent despite his initial summoning. This means that if Xanxus summoned Bester without being angry and was later angered that Bester's stripes would show and his energy would increase in direct correlation to Xanxus' mood.
Bester seems to have a distinct will. In his first appearence he sticks to the shadows and protects Xanxus. I like to think of this as more than just what Xanxus had wanted despite the fact that he can be a manipulative bastard. Bester is also fiercely destructive; something that I am certain is not exclusive to his flames or master. Xanxus never actually gives Bester commands; instead, he simply calls his name when he wants something. They have a very deep understanding, as well they should. I feel that Bester is most likely one of the few box weapon animals that could (and does) walk around freely doing whatever he wants. Since he lives based on the amount of energy the wielder can expend, I feel that Bester would be close to immortal.
Bester is more of a pet or companion to Xanxus. Xanxus trusts that with a simple calling of his name that Bester will do everything according to Xanxus' will. In fact, Bester is most likely the only thing under the big blue sky and the life giving sun that Xanxus trusts. This is most likely because Bester survives on the basis of Xanxus' own life force, making Bester a direct extension of Xanxus. It also explains the similar mood swings, temperament, and cat naps. It is my belief that because box weapon animals are sustained via the life force of the user that they take on many traits of their master. I also believe that because pets tend to take on traits of their owners that Bester is most likely an extremely bad tempered guy that just wants everyone to shut their mouths and let him nap. Besides, Xanxus is bad tempered, lazy, and fickle enough to be a cat.