You can see in darkness (shades of gray) up to 30ft

you can use your tail to jump and push up to an extra 5ft, and makes it easier to do athletics checks.  

you are slightly faster then the normal human being, due too your small size and  demonic origin. 

you have a natural proficiency for persuasion and deception. guilting is your specialty!   

you have an added bonus to your charisma! 

you must be bound to an important object of your choice, from major landmarks, to vases and necklaces of high value. 

due to this, when you die, there's a week long window for when you can be pulled from the underworld by summoners. to be summoned, they must take your heart, and force-bond it too an object. it does not have to be of great value, they just have to engrave it with your name, and placing it near/inside the object, then they must cast a healing spell upon the object. 

finniker hearts look like very shiny rocks. of course, every time they are revived, they come out a little differently. they may be physically harmed, or their personally may have changed drastically! in any circumstance, both parties need to be very careful. anything can emotionally break a finniker shortly after summoning.