Harem Plan

The Harem Plan (楽園計画 Rakuen Keikaku, lit. "Pleasure Garden (Paradise) Plan" or ハーレム計画 Haremu Keikaku, lit. "Harem Plan") refers to a plan that was initially proposed by Lala and her costume robot Peke to Rito after his confession of love to Lala after explaining about the legal situation of Terran polygamy. It has since been revised and expanded by Momo and has made steps to fruition since its inception.

Justification and Plausibility
The only resistance to the Harem Plan on Rito's end is grounded in an unwillingness to break the law; polygamy is outlawed in Japan, so a harem would go against it. A common justification for the Harem Plan given in response by the series' Devilukean characters - as well as its biggest enabling factor - is the sovereignty of the planet Deviluke over the Milky Way Galaxy and also perhaps the known universe: It is highly unlikely that its ruler - even if a foreign one - would be challenged given Deviluke's status as an intergalactic superpower. Thus, if Rito were to marry Lala and become its next king, then he would be diplomatically immune to Terran legal procedure; the laws of Earth would not apply to him, being null and void, essentially freeing him to marry whomever and however many he chose.

The Original Harem Plan
The first incarnation of the Harem Plan was first incepted by Peke in response to Rito's above-mentioned opposition to taking more than one wife. Since Rito is also in love with Haruna, Lala decided to share him with her best friend. The goal of the original plan was to get Rito to confess his love to Haruna; as Lala knows of the latter's feelings for the former, success would be guaranteed as there would be no reason for either of them to reject the other's love.

Revision and Expansion
In To LOVE-Ru Darkness, it was revealed that Momo is also in love with Rito, but does not want to intervene in Rito and her sister's relationship. However, when Lala told her about her Harem Plan, this gave Momo hope for a compromise where Rito could love her as well. She created a revised and expanded version of Lala's Harem Plan, which includes herself and every girl currently showing romantic and/or sexual interest in Rito, so that all of them (including herself) may be happy without hurting anyone.

The goal of the revised Harem Plan is to have Rito marry Lala as his legal wife and then have Haruna as his concubine, with every girl who has feelings for him as additional concubines. The girls included in Momo's version of the Harem Plan are herself, Lala, Haruna, Nana, Yui, Yami, Run, and Rin, with Kyouko Kirisaki, Tearju Lunatique, Mea and Risa under consideration to be part of the plan. Mikan is also included, despite being Rito's sister. In the anime, many of Momo's imagination arts have Mikan removed from them compared to the same art from the manga, possibly to avoid getting too high ratings for the anime.

While Momo reasons and believes that the Harem Plan is the perfect solution to guarantee the happiness of all of Rito's love interests, there is a hint of greed in her plan of using other girls to make a harem for Rito so that she may be included as well, as she doesn't care if she is second or third as long as she receives Rito's love. She is including not only established love interests but she is also on a lookout for potential candidates to include as well. She may even consider girls who are not even love interests yet, such as at the end of To-Love-Ru Darkness Chapter 32.5, when she wrote down the names of two of Mikan's friends, despite only having met Rito for the first time. She intends to include as many girls as possible because she feels that the more girls fall in love with Rito, the more difficult it would become for him to reject them all, thus he would more likely accept her plan as a logical solution.

As the series progresses, the plan continues to advance little by little. However, shortly after her arrival, Nemesis tells Momo to give up on the plan because she wants Rito all to herself as her servant. It's unknown if Nemesis is still an obstacle to the harem plan. She no longer seems to want to take Yami and Mea back into space with her, but instead lingers in the city to bide her time, visiting Rito on occasion for her entertainment. Mea on the other hand, intends to help Yami and Nana become more open with their feelings and thus decides to help advance the Harem Plan much to Momo's reluctance.

Although she still continues to advance it by encouraging Rito's relationships with the other girls, Momo has recently experienced doubts about whether or not she would be happy with the Harem Plan. But after hearing Lala's unfaltered satisfaction of just being part of Rito's life, she is once again reassured and optimistically continues to advance Rito's relationships with the other girls.

In Darkness Chapter 73, Rito is seriously considering to accept the plan, as it seems to be the only way. He has a dream about himself accepting the plan and becoming the king of Deviluke with Momo alongside him. He also asks Lala if she'd be fine with him loving other girls beside Lala and Haruna. Lala tells him that she'd be fine if it'll make Rito happy.

Trivia

 * In both versions, the Harem Plan depends on Rito and Haruna confessing their mutual love for each other.
 * However, a possible impediment to the revised and expanded plan comes in the form of Yui Kotegawa and her opposition to anything she considers "shameless". As Yui wants Rito to only engage in "shameless" acts with her and no one else, she forms another obstacle to the plan.
 * The two versions of the Plan have opposite motivations: The original plan advocated by Lala and Peke was motivated by love and a desire for the happiness of everyone involved. In contrast, the revised and expanded plan devised by Momo, while also motivated by love, is also motivated in small part by greed. The revised and expanded plan is also pushed along by insecurity, as Momo is concerned that Rito would not love her under any other circumstances.