Hi so I'm going to sit some ground rules/characteristics

They are weak to radiant damage(1.25x), can shapeshift, are bound to an object or person, they share the same magic/energy pool with the object/person they are bound to (no pool no magic), the shadow can't be killed directly if the reach 0 hp they will be gone until (object/person will not have a shadow) next dawn for object or a long rest for person, they with die if object they are bound to is destroyed or the person they are bound to dies. 

So please reply your ideas for pasts, roles, alignments, goals and stuff.

Thank you for giving you time

Okay. So the way I figure, there's three distinct ways of looking at this. Let's see where this goes...

1. The PC is a member of a species that is somehow connected to the shadow realm. Maybe it's a bloodline thing - like a Shadow Dragonborn. Maybe it's a fey thing - like a dark fey from the Shadowfell. So we should balance this against the starting modifiers for the other available PC races. So I'd figure maybe a +2 to Dexterity and a +1 to something else, like maybe Wisdom. Bonus proficiency in Stealth. Darkvision to 90 feet. Vulnerability (double damage) from Radiant damage. Disadvantage on rolls when in direct sunlight. And there should be one other cool thing, like maybe "once per short rest when you're hit by a melee attack you can use a reaction to reduce the damage by half and instantly bamf to a spot within 60 feet (that you can see) that's in shadow or darkness."  Or maybe just "once per short rest you can cast the Darkness spell without using a spell slot". And if the PC is a shadow dragonborn, their breath weapon is a 15 foot cone of necrotic energy. 

2. The PC was originally some other race (like human, elf, dwarf, etc) but has been cursed into a semi-undead shadowy existence. So we would use all the starting modifiers for the original race, with a few minor adjustments. The PC still gets the two drawbacks - vulnerability to Radiant damage and disadvantage on rolls when in direct sunlight. Also - the PC can be turned by a cleric's Channel Divinity ability! To balance those, they either gain 60 foot darkvision or they add 30 feet to their existing darkvision. They gain resistance to Necrotic damage. They also gain an Undead Fortitude ability (once per long rest if damage would reduce you to zero hit points you instead drop to one hit point, but not if the damage was Radiant or a critical hit). 

3. The last option is a bit... different. The PC is not actually a real person. They are instead merely a shadow of someone else. This could be a bit tricky to play, but it would give the player and their DM some really cool backstory hooks to mess around with. So maybe the original PC died with unresolved issues, so their shadow still walks the lands looking to set things right so they can rest. Like maybe the original PC was cursed by a BBEG and now their shadow has to defeat that BBEG so their soul can rest. Maybe the PC was trying to create a phylactery for some immortality experiment but something went horribly wrong and now the phylactery is missing, so their shadow searches the world for it so they can return to normal. 

So you could play this the easy way - which is similar to #2 above, except that you can't be turned by clerics. But if you really want to make your own life more difficult, try this: Your very physical form shifts in and out of the material plane, depending upon whether you are moving toward your End Goal or not. If you're getting closer to the BBEG, or to your phylactery, or whatever, then your body has physical form. BUT! If you're not moving toward that goal, your form loses material cohesion and you become a shadowy ghost of sorts. Sure, this might sound cool at first. You can't be hit by non-magical attacks. You can fly at your regular movement rate. You can squeeze through a 1 inch hole with no problem. You are effectively invisible while in darkness. You can't be grappled, or knocked prone, or restrained. Sounds great! Except that your attacks also can't do any physical damage. And you may have to make a Wisdom saving throw just to pick something up and hold it, or to open a door. But there's an even bigger catch... each time this happens you have to make some kind of saving throw or you lose a little piece of your soul. When you run out of pieces, you disappear completely and drift off into the shadowy void of nothingness beyond this world. You are now an NPC. 

Sure, your DM may have to fudge up some homebrew mechanics for this. Like maybe at the end of each day of not moving toward your goal you have to make a Wisdom saving throw. The DC starts at 8 and goes up by one every day until you fail a saving throw. On a failed saving throw your Wisdom Score drops by one point and the daily DC gets reset to 8. Eventually... inevitably... your Wisdom will drop so low that it'll be impossible to pass the saving throws. So you get to have some fun being an incorporeal spookyguy, but you know that there is a mathematical progression at work that is just counting down the days until you just no longer... are. 

So that's my two copper pieces at least. 

Have fun!

Than you for the reply. all your ideas were great. Three was amazing. but I was think a person that is stuck with a living shadow. Number ones race bonus are all good but the idea of this thread was to see character ideas like a shadow maybe ruining the person it's bound to life. Over protective of a powerful magic sword holding it back behind them. If the original idea is to powerful maybe shadows lower charisma. When the shadow is gone all rolls except for charisma are halved. Rather sad bonus if the object is mended or the person is revived the shadow needs to be revived separately I think you can see the implications. Please leave a reply thank you all. All 82 as of now viewer thank you for giving your time

Maybe a vampire's shadow was cast off from their body after they were bitten. Now they shadow wants to reunite with its body. Was that the kind of thing you were asking about?