Copy allows Neito to utilize a Quirk after coming into contact with its respective user.
Neito only requires to touch as little as a few strands of hair from the target in order for his Quirk to take effect. After contact with a target is made, Neito gains full access to their Quirk, at least at a basic level; it's unknown if Neito is also capable of copying further improvements that one may achieve with their Quirk.
Copying multiple Quirks at once is possible as well. Neito can hold up to a certain amount of copied Quirks within his body, depending on how much he trains his Quirk. The effects of the copied Quirks, save for those that physically change the structure of his body, will continue to remain active even if Neito switches out the associated Quirk. Size, for example, will have its size-changing effects persist on an object Neito has altered, even if he’s since switched to another Quirk.
With a copied Quirk, Neito essentially possesses the same advantages and disadvantages of the original Quirk user he copied it from, although his ability to swap between Quirks makes him a more versatile and unpredictable opponent to deal with in group battles, where he has more opportunities to copy numerous, different Quirks, be it from friend or foe.
There are a number of drawbacks to Copy. This Quirk requires Neito to make physical contact with another Quirk user for it to function in the first place. He is unable to activate more than one copied Quirk simultaneously, which means he's instead forced to switch between them.
Neito has a time limit of how long he is able to keep a copied Quirk stored within his body, which was initially stated to be 5 minutes; however, Neito appears to have improved this limit, stating that copied Quirks can now last up to 10 minutes. After this limit is up, Neito loses access to the copied Quirk until he makes contact with its original user again.
While Neito is capable of copying any Quirk, those that depend on accumulated resources as a prerequisite condition to function correctly can't be used as intended, since Neito is able to copy Quirks at their default states only, having no way to replicate the quantities of whatever the Quirk needs to work properly through Copy alone. This means, for example, that he's incapable of tapping into One For All's strength, as it's the result of power that's been stored throughout the years thanks to its stockpiling trait.
As noted in Neito's backstory, he had a hard time justifying his goal to become a Pro Hero to other people due to his Quirk being useless by itself. As a result, Neito dismisses conventional heroism and is willing to employ underhanded tricks and tactics in order to gain the upper hand against stronger opponents, especially if it helps him acquire their powers.
Neito was able to improve some of Copy's limits since its introduction in the U.A. Sports Festival. Neito is capable of holding up to four or more Quirks at once instead of three, an assumed number going by the amount of watches he carries around on his hero costume, which, as a result of this, is something that Neito is capable of exploiting in order to deceive observant enemies.
Neito seems to instinctively know how to utilize a copied Quirk, even if it's his first time seeing it. So far, Neito has been shown copying the following Quirks:
- Solid Air
- Softening 
- Brainwashing (Presumably)
- One For All (Failed usage due to lack of necessary resources)
- Twin Impact
- Rewind (Failed usage due to lack of necessary resources)
- Fat Absorption (Visual example)
- ↑ 1.0 1.1 My Hero Academia Manga and Anime: Chapter 29 and Episode 18.
- ↑ My Hero Academia Manga: Chapter 215.
- ↑ 3.0 3.1 My Hero Academia Manga: Chapter 217.
- ↑ My Hero Academia: School Briefs II.
- ↑ My Hero Academia Manga: Chapter 214.