Nikki has methods to copy enemies' abilities, like ripping a page from a wizard's spellbook and adding it to her own, removing a robot's plasma cannon arm and use it to upgrade her BFG, and being a master martial artist, which allows her to copy any move after seeing it a few times.
Nikki is an AI, and she is capable of analyzing the code of enemy AI and integrating it into herself.
Inverted: Whenever Nikki/Mike defeats an opponent, she/he loses the ability that was used to defeat that opponent.
Subverted: Nikki knocks out a star swordsman after an epic fight, and gains... The ability to communicate with plants.
Double Subverted: Confused, Nikki questions the swordsman after he comes to, and he admits he used to be an earth mage with that exact ability.
Parodied: Whilst driving home, Nikki accidentally runs over a possum in the road. She subsequently discovers she now has the power to play dead. (Later on, she uses this ability to fake defeat and set up a cruel Kaizo Trap for a particularly tough foe.)
Zig Zagged: Sometimes the character gains skills from battles and sometimes not.
All Nikki's and Mike's powers are their own.
They learn new abilities as they gain levels and advance through the game.
Enforced: "Why would we bother creating separate set of abilities for the heroes and for their enemies? Just make the heroes capable of copying their enemies!"
The Big Bad sends out a slew of opponents with the ability to self-destruct. When Nikki defeats them and habitually steals and tries out their powers, the results aren't pretty... only it turns out that our hero had gotten the ability to duplicate herself earlier on, and the Nikki that just self-destructed was one such clone. Nikki uses this "Clone Bomb" technique to take down many enemies.
Nikki goes on a rampage defeating as much opponents as she can to get their powers.
Nikki finds a shapeshifter, defeats them, and gains access to otherwise useless abilities.
Nikki has limitless physiological and cognitive functions because one of her acquired powers is Super Intelligence.
Played For Laughs:
Nikki is so focused on looking for abilities to acquire she reaches Memetic Molester status.
Nikki basically ignores the powers she picks up this way, because if she didn't need them to stomp all over the person who had them and she took them from, they can't be that powerful in the first place, right?
Basic Trope: Gaining new abilities by being hit by them, and learning them in the process.
Straight: Whenever Mike is hurt by an opponent's signature attack, he learns the attack and can use it in any opportunity.
Exaggerated: Whenever an enemy hurts Mike, he copies every skill they have ever accquired.
Downplayed: Mike is Awesome by Analysis, and can usually imitate an opponent's moves - but he still needs to practice if he wants to get any good at a given technique.
Justified: Mike's Congruent Memory allows him to learn skills by seeing a demonstration.