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 must actively switch between them on the fly.
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 it's original user again.
While Neito is capable of copying any Quirk, those that depend on accumulated resources as a prerequisite to function can't be used as intended, since Neito is only able to copy the Quirks themselves, with no way to instantly replicate any additional attributes or conditions it needs to work properly. This means, for example, that he is incapable of tapping into One For All's strength, as it is the result of power that has been passed down and multiplied over the course of decades thanks to its stockpiling trait.
He is also hindered by the natural limits of his own physical body; while he can properly adapt Quirks that specifically augment his body in some way (such as Hardening's defense boost and Explosion's immunity to the blasts it creates), he would not instantly have the strength necessary to properly handle powerful Quirks that require years of physical conditioning to use without bodily harm, such as One For All.
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 
- Erasure 
- 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)
- Half-Cold Half-Hot