I have a player who wants to essentially play Kakashi from Naruto.  I have no knowledge of the series.  At level 1, he wants a sharingan eye.  My big problem is that I don't understand the properties of said eye.  Is there an item in DnD that can function similar to it?  I can't really find anything that explains what the sharingan eyes actually do.

Just a quick google gives you wiki articles on this hosted by the same company that owns D&D Beyond.  My quick take, the eyes have properties that would be way overpowered for level 1 (there's basically insightful fighting as well as skill advantages) and some that don't seem to have any mechanical resonance in the game (like mirroring an opponents jitsu, that's probably best represented as insightful fighting or you could homebrew something where a PC can immitate a fighting style if a specific fighting style is represented (or adapt an opponents proficiencies etc).  I won't even start on the hypnosis powers.

My advice would be to explain how a level one game starts at a much lower power level than the principles of Naruto; but you could see a rogue build as a way to get there, and say that the eyes themselves are likely going to have to be earned as a feat or boon over time.  These eyes are earned in the Naruto world after a particularly powerful event, an event of the sort a level one character is unlikely to have experience ... yet.  

Thanks for the info.  It was mostly the mechanics of the eyes that I couldn't find clear insight on.  I always found articles about the series Naruto itself.  Not what the sharingan eyes do.  If I could've understand the eyes, I could better put it into DnD terms.  My player has no knowledge about DnD.

Quote from Flamehair >>

Thanks for the info.  It was mostly the mechanics of the eyes that I couldn't find clear insight on.  I always found articles about the series Naruto itself.  Not what the sharingan eyes do.  If I could've understand the eyes, I could better put it into DnD terms.  My player has no knowledge about DnD.

 At their most basic level, they let the user predict what the opponent will do based on how their muscles twitch and the energy within them moves and copy them perfectly, as well as giving bullet time vision. It would be as if a wizard could copy their opponents spells after seeing them once or a fighter could see what the enemy will do before they do it so that they can prepare their counter/dodge beforehand. They also let the user hypnotise people who look into their eye. It has three normal levels of power, with it becoming increasingly more effective with each.

At higher levels independent of the previous three (after the user suffers trauma related to causing the death of someone they love), the sharingan can do more esoteric stuff like set on fire everything within sight with a special fire that only a sharingan can turn off, putting the opponent into an illusion with time dilation and creating a giant spirit armor around the user. Kakashi's could teleport stuff to other dimension and if something or someone was not entirely inside the area it tore it apart.

Kakashi was not born with the Sharingan, but it was implanted on him. The lack of compatibility meant that he tended to suffer exhaustation after short use. It was still enough for him to be known as someone who copied 1000 jutsu (basically spells).

Naruto is a pretty popular series though. There probably is an adaptation that breaks down sharingan skills by character level.

Edit: Ok, if I had to make a Sharingan user... I would probably make a bladesong with some kind of hypnosis spell or the level two skill from the enchantment specialization and the ability to copy spells as long as you have an apropriate level spell slot free while you had the sharingan active. Maybe give a level of exhaustation if that skill is used since getting exhausted is typical of Kakashi and it might help balance out. Just tell him that spell slots are akin to his chakra reserves and that he can't copy jutsu or techniques that he doesn't have enough control to cast yet.

I also forgot to mention that sharingan gives eiditic memory of things that you see with it and that since you can copy the movements of those you see it is good to read lips and copy handwriting.

Edit^2: I also forgot it let's you see through illusions...

So a loyal imterpretation of Kakashi's Sharingan would be something like: As a bonus action, activate Sharingan (lasts 1 minute). It grants the benefits of Truesight except seeing in the dark and looking into the etereal plane. (so mist and darkness limits its effects but not illusions or transformations). As long as you can see your opponent, you get a bonus to your AC (probably your spellcasting modifier), advantage to attacks and can add the spells you see that are in your class list to spells known/prepared/spellbook. Once the sharingan is deactivatef you gain a level of exhaustation.

Quote from Flamehair >>

I have a player who wants to essentially play Kakashi from Naruto.  I have no knowledge of the series.  At level 1, he wants a sharingan eye.  My big problem is that I don't understand the properties of said eye.  Is there an item in DnD that can function similar to it?  I can't really find anything that explains what the sharingan eyes actually do.

Talk with the kid and ask him the total list of powers, explain to him the Naruto can be a wee bit overpowered for the game and that you need to balance it out with the other characters.  From there, look up the monks and start building a custom class.  Also tell him that if things start going haywire you'll have to rework the class as well.  And no, sharingan Eye at level probably not a great idea.  Maybe have it be usable once per day and advantage on an attack or force an attack that would hit to reroll.  Maybe increase its use slightly as the game progresses if he wants it right at level 1.

Lol, the sharingan is one of the most OP things in Naruto. A fully functional Kakashi sharingan eye is going to blow your campaign balance to pieces at lower levels.

As others have said, Kakashi's abilities with his eye allow him to copy moves he's seen and replicate them perfectly (meaning he can copy attacks by watching them once). He has heightened reflexes and can see through illusions. As the series progresses, he can warp reality with his eye, causing whatever he focuses on to be ripped from this plane to a demiplane from which there is no escape except when he allows it. Finally, he can eventually transport himself and other objects/people into and out of that demiplane at will, as long as he has the stamina for it. The sharingan requires a ton of energy from Kakashi, though, and when he uses it early in the series, he's out of commission for days. 

I second the advice to ask your player what abilities he's hoping for. Kakashi is a hard build to translate to D&D (believe me, I've been wanting to do this myself for years), but focusing on certain elements can make it easier. As for the eye itself, you could give him an Ersatz Eye and homebrew some gradually scaling features for it. Maybe he gets darkvision from it, or he can use it once per long rest for advantage on initiative rolls or one attack roll/saving throw. Or you could juice it up and make it more powerful (grant an Action Surge, or give him something like a divination wizard's Portent rolls) but require him to take a level of exhaustion after using it until he's gained a few levels. Just some thoughts.

So I got something to at least run by y'all.  At levels 1-4, it is just a basic Ersatz Eye.  But starting at level 5, you can use the eye to force a successful attack to be rerolled once, taking the second roll, even if it is higher.  But each use of this skill gives a point of exhaustion.  Then at level 12, you can take an Action Surge at the cost of 2 points of exhaustion.  Then at level 18, you can cast Hypnotic Pattern without the verbal, material or somatic components for 2 points of exhaustion.  And the range of this spell is reduced to 30 feet.  Figure that if he made it to the higher levels, which I doubt he will, this might give him a way to bow out if he wants.  A bit of a bs throw away thing that I will be able to explain away.  Maybe by having it be destroyed upon death.

Maybe early on, give the player a sense that anime flourishes mechanically play out as extreme recklessness in your world.  Like have a beloved or at least liked NPC go full on Naruto running at a perimeter and have them instakilled by some Area 51 style perimeter defenses ....

I kid.

Quote from Flamehair >>

So I got something to at least run by y'all.  At levels 1-4, it is just a basic Ersatz Eye.  But starting at level 5, you can use the eye to force a successful attack to be rerolled once, taking the second roll, even if it is higher.  But each use of this skill gives a point of exhaustion.  Then at level 12, you can take an Action Surge at the cost of 2 points of exhaustion.  Then at level 18, you can cast Hypnotic Pattern without the verbal, material or somatic components for 2 points of exhaustion.  And the range of this spell is reduced to 30 feet.  Figure that if he made it to the higher levels, which I doubt he will, this might give him a way to bow out if he wants.  A bit of a bs throw away thing that I will be able to explain away.  Maybe by having it be destroyed upon death.

 This feels oddly underpowered and kind of punitive to me, given that an Ersatz eye takes up an attunement slot and adds nothing mechanically to a build. Waiting until level 5 to grant second-rate Luck rerolls also makes it not really worthwhile to use considering the heavy penalty imposed. Until the party is level 9 and can cast Greater Restoration (which costs 100gp in diamonds each time), the only way to get rid of exhaustion is by completing a long rest - and that only removes a single level of exhaustion. In addition, the use of the eye could make the situation worse for the player if the second roll is a crit that they are forced to take. 

This player clearly wants to feel cool and have access to one of Kakashi's signature features. For Kakashi, the surge of battle prowess was worth the tradeoff of the ensuing exhaustion. I think this proposed eye is more likely to turn the character into a liability, as exhaustion can quickly become debilitating. And it's not fun to play a character who's a liability.

Personally, I don't think an Action Surge in tier 1 play is game-breaking, especially if it comes with a toll. I can understand if you aren't comfortable granting the player special abilities just because the character is patterned after a powerful anime character, though. At that point, it might just be better to tell the player they can have a sharingan for flavor purposes but that it won't impart any special abilities. "Sorry, those abilities don't translate well to D&D" is a valid thing to say to someone who's new to the game.

In order to keep game balance, you don't want to give out permanent magical items until the characters hit level 6. If the party find a couple of +1 weapons somewhere between level 3 and 5 that's not going to break the game, but avoid giving out powerful items at low levels.

If you really want to, then homebrew an item that allows the PC to take advantage on an attack roll before the attack is rolled - this mimics being able to predict their movement. It still doesn't mean he's good enough to hit them.