A character becomes Harem-Powered when some or all members of their harem of Love Interests (wanted or otherwise) grant them powers, equipment, abilities or immunities. This can be in general (granting Life Energy to the central character) or very specific (granting one unique item/ability). In narrative terms, this trope typically acts as an unspoken justification for why the character can't choose only one partner, as rebuking any member of the harem would affect their optimal combat ability. In some cases, the character may only be able to access one power at a time, but still needs to keep members of the harem around in case they need that specific power at a specific time.
In video games, this is related to Level-Up at Intimacy 5.
- In Mahou Sensei Negima!, Negi spends much of the series slowly amassing "Pactios" with the students of his class - a magical contract in which a "Magister" empowers a "Minister" to act as their partner in battle, and creates a Pactio Card with which the Minister can summon a powerful artifact matching their personality. Just before his climactic battle with Jack Rakan, Negi enters a temporary Pactio as Princess Theodora's Minister, receiving the artifact One Thousand Bonds - a cardholder which can summon the artifacts of any of his own Ministers, and is referred to even in-universe as a Story-Breaker Power.
- Trinity Seven: Amongst Arata's plethora of powers and abilities, he possesses Power Copying, so long as he follows certain prerequisites to using an ability. His Battle Harem thus often takes it upon themselves to teach him to use their abilities (such as Full-Contact Magic) to stand a chance against the latest Arc Villain. He is also in possession of multiple Grimoires and even created an avatar for Lilith's Grimoire that has a mix of both "parent's" magic, which he can tap into at any time if his is depleted.
- 12 Beast: Eita already has implausible ninja training, but gets superpowers when any of the girls he travels with fuses with his metal gauntlet (the harpy gives agility, the minotauress gives strength, etc.). Unfortunately, this involves him putting his hand on their breast.
- Vandread: Hibiki gains access to a Mecha that can combine with the mecha of several of the female heroines. The girls fight over who gets to combine with him and each fusion grants a different and specific power to the resulting "offspring". In the final battle, the mecha of the hero and all of the women combine into the "Super Vandread", which has all of the powers of the various fusions.
- Youkai Shoujo Monsuga: The various Youkai girls can project their spirit into Yatsuki to grant him weapons or armor themed after their abilities. Initially he's limited to one at a time, but after figuring out how to fuse his soul with theirs he's able to equip all of their powers at once.
- Prétear has its heroine combine with one of her Leafe Knights in order to activate one of the Pretear's eight forms.
- Asura Cryin': A regular human is able to access two types of power.
- By sacrificing the physical body of a partner, they can draw power from their bonds/love and use them to power a Humongous Mecha called an "Asura Machina". The partner in question literally has their emotions drained to supply energy to the mech and its attacks. Eventually, that partner will lose all emotions, run out of Life Energy completely, and disappear.
- By forming a "contract" (usually a romantic/sexual relationship) with a demon, humans can create a Familiar called a "Daughter". The demon and the Daughter will gain power from the contract, but it will not only drain Life Energy from the demon (who will crumble into crystal once depleted) but also slowly erode the human's ability to love.
- The only way to circumvent the energy issue is by taking BOTH of these options and creating what's called an "Asura Cryin". This allows the Machina and the Daughter to circulate magic with each other, both increasing their power and slowing down (but not stopping) the energy depletion. However, this does NOT solve the emotion/mind drain problem. This is reason is why Asura Cryin are feared and typically killed on sight: they're a nigh-unstoppable Person of Mass Destruction who's had their empathy and mental state eroded to the point of insanity.
- In addition to this, Tomoharu ALSO gains powers from Nia, a demon who has luck-granting/draining abilities, which she can bestow upon him with a kiss.
- Campione!: Godou's most powerful ability, which is essentially an Instant-Win Condition, can only be activated once he sufficiently understands a god, as well as their powers and origins. The Girl of the Week is usually the only person who has that knowledge, which would take years of explanation to cover verbally. It's possible to impart knowledge all at once via magic, but a Campione's skin is Anti-Magic. Thus, the only way said girl can give the hero the power he needs to save the day is to insert the magic inside his body, which typically means through kiss or you-know-what.
- Date A Live: Shido can absorb the powers of any Spirits he romances with a kiss so they can have normal lives. Over time, he gains a measure of control over his powers and can develop rudimentary versions of the Spirits' magical weapons for his personal use. This is because the First Spirit resurrected Shido as a Half-Human Hybrid when he died before the events of the story. The collective powers of the Spirits were always meant to be Shido's and the First Spirit merely turned regular humans into Spirits as a way to perfect and refine those powers until Shido eventually obtained them.
- In The Hidden Dungeon Only I Could Enter, Noir gets Life Energy from erotic acts with the opposite sex; however, this loses effectiveness quickly if he only plays around with the same girl over and over again, so he needs a harem to "spice things up" and keep that LP flowing, or he'd die.
- Hundred: The trope is Played With, in that Hayato gains a Power Limiter through his harem because he's a "Variant"—a Half-Human Hybrid caused by his DNA being infected with the Savage virus. Using too much of his Variant power can cause him to go berserk, unless the Savage virus is put under control. He can do this by "giving" some of the virus to someone else, via a kiss. If the person wasn't already a Variant themselves, they will become one by kissing him. In addition to this, two other women in his harem (Sakura and his little sister Karen) can also calm out-of-control Variants with their music.
- In Another World with My Smartphone: Touya can use any spell ever created, meaning he can use spells specific to any mages in his Battle Harem, and even combine them to create Required Secondary Powers for more complex abilities, such as Super Mode or Flight. He also gains access to the island of Babylon, which is a base created by Professor Regina Babylon to aid him in his upcoming battle with the Fraze with access to an alchemy lab, a mass-production workshop, and facilities for producing Humongous Mecha. Due to Regina's nature, each piece also comes with a Sex Bot for Touya...including Regina herself.
- I Saved Too Many Girls and Caused the Apocalypse is a combination of this and Story Breaker Teamup - due to being dragged into multiple stories from different genres, Rekka gets to use abilities and equipment in places where no one sees them coming. Can't pull out the Sword In The Stone? Just borrow the alien girl's teleportation device!
- Magika Swordsman And Summoner: Kuzuki Hayashizaki is granted by his contracted summon Lemegeton the ability to use the magical powers of the girls in his harem. The more he and a girl love each other (Lemegeton clarifies it has to be love, not just friendship), the more powerful spells he can access.
- Rokujyouma no Shinryakusha!?: This is a very downplayed trope, played in a more practical, long-term method than usual. Koutarou finds himself in situations where one of the girls in his harem teaches him some type of skill or bestows upon him an item or skill that slowly starts to turn him from an Ordinary High-School Student into the spitting image of the legendary Blue Knight. This allows Koutarou to eventually become the Blue Knight (via Time Travel). A full list: Sanae alters his body so he can see and manipulate spiritual energy, Theia gives him Powered Armor and the high-tech sword Saguratin, Ruth trains him in sword-fighting, Yurika gives him a defensive enchantment, Kiriha gives him a gauntlet that produces fire and electric attacks, Clan maintains his armor and incorporates the gauntlet into it, Alaia gives him the magic sword Signaltin, either Alaia or Harumi are needed to fully use the power of Signaltin, and Maki creates a mental link with him. However, Koutarou doesn't feel like he deserves the respect given to him due to his power coming from others.
- Sekai No Owari No Encore: Due to his resemblance to an ancient hero, Ren ends up travelling with the hero's old Battle Harem - an angel, a demon and a dragon in human form, who each give him a minor boon so that he won't get himself killed by enemies who mistake Ren for the hero himself.
- Strike the Blood: Kojou is a Pregenitor vampire with the ability to summon highly destructive familiars. However, the familiars will not respect a "virgin" vampire and will only awaken if he drinks blood. In addition to that, some of his familiars require multiple different "blood donors", some require specific types of blood (or Life Energy) and some just become bored from receiving blood from the same partner over and over. Because of that, Kojou is required to keep a harem of women (and possibly men) to access his powers.
- Unlimited Fafnir: Yuu is a "D", or a human born with the powers of a "Dragon", enormous Kaiju that select humans as "mates" and turn them into a matching "Dragon". Except later it's revealed that he's actually a "Counter-Dragon", and he also has the ability to "mate" with Ds. He thus "mates" with every girl in his school to protect them from being claimed by other Dragons.
- World Break: Aria of Curse for a Holy Swordsman: In a world where people gain power from Past-Life Memories, Moroha is a rare example of someone able to gain power from two past lives. In one past life, he was a swordsman fighting for his little sister and in another, he was an Arch Mage leading a slave rebellion with his wife. All three of them have been reincarnated into this era, and only through deepening his relationships with them can he remember his past abilities. And on top of this, he also has a regular Battle Harem which provides backup.
- Conception: The main character Itsuki is a "Visitor"—a Summon Everyman Hero tasked to fight the "impurities" which threaten it. To do this, he must romance twelve (later thirteen) maidens of virtue who will bear his "Star Children". Actually, mating isn't a requirement of the experience, but it does involve a lot of G-Rated Sex. He must "conceive" as many Star Children from each of the women in order to have an army strong enough to clear the impurities.