Ren Elsie Jewelria

Ren Elise Jewelria is a childhood friend of Lala and the boy version of Run Elise Jewelria. As Ren and Run are royalty from planet Memorze, they can change gender in mind and body upon triggering a specific stimulus; in their case, a sneeze. Ren likes Lala, while Run likes Rito. Ren hates Rito because Lala likes him, while Run hates Lala because Rito likes her.

In Chapter 9 and episode 6 of To Love-Ru Darkness, Run and Ren permanently separated into two separate beings after reaching 'adulthood'.

Appearance
Ren has hot pink eyes and the top of his hair is white with two points sticking out, while the bottom back part is black, during his first appearance many girls  took a liking to his looks.

However in the To Love Ru Darkness manga Ren is shown to have blue hair.

His hairstyle is similar to Chaud from the video game series MegaMan Battle Network.

Personality
Ren's personality can be described as determined, but stubborn (especially where Lala is  concerned). In the first series of To-Love-Ru, he constantly tries to prove that he is a real man to Lala (who passively promised to marry him if he became more manly, although she doesn't recall it) by competing against Rito to pointless challenges. Ren would overly complement on Lala's beauty and her kindness towards others.

In To-Love-Ru Darkness, Ren's appearances become less frequent, but he seems to have become more calm and mature. He doesn't seem to try to impress Lala or try to compete against Rito anymore, so it's possible that he has accepted him as Lala's lover, although Ren still shows his love for her.

Childhood
Sometimes Ren would be taken to Deviluke to play with Lala. Despite all the trouble she put him through with all her experiments and mischief, he apparently falls in love with her and cherishes all of those memories with her. However, his female counterpart remained angry at Lala for all of the abusive fun Lala had with them. Ren one time asked Lala if they would get married if he becomes a real man, which she agreed to but doesn't remember it.

Powers and Abilities
It is said that due to evolution, those from Memorze like Ren are born with both male and female genders, each with separate personalities. N ormally, Memorzorians would change monthly, but due to the differences in environment and magnetic fields on Earth,  Ren and  Run  change after a sneeze, and both have different opinions about certain things. Ren is capable of com municating with Run within his subconscious while in control (this also works for Run). After reaching adulthood, they both physically separate, and Ren no longer shares a body with Run, so this ability is not usable.

Ren is shown (comically) to have great speed and strength. In episode 12 of the original anime series, he is seen running really fast even while carrying a really huge weapon, but was unable to control his speed while carrying it.

Lala Satalin Deviluke
Ren and Lala have been friends since childhood. Lala would occasionally force Ren to test out her inventions and make him where dresses, but despite all this, he didn't seem to mind in the end because was deeply in love with her (but Run grew angry at Lala). Lala agreed to marry Ren if he proves that he has become a real man, although she doesn't remember this because of her lack of attention, but Ren still tries anyway to win her over.

In To-LOVE-Ru Darkness he is hardly seen making any effort to try and impress her anymore, so it's unknown if he has "accepted defeat" to Rito or still pursues her off scene, but he is shown to still care about her.

Rito Yuuki
Ren sees Rito as his rival because of Lala's love for him, so he constantly tries to best Rito on who's more manly, even at completely pointless challenges. Ren once accidentally kissed Rito, which caused Ren to despise him even more, but caused his female counterpart to fall in love with Rito, which Ren blames him for.

In To Love-Ru Darkness, Ren seems to rarely be seen trying to act manly to impress Lala and challenging Rito anymore (it's unknown if he has accepted Rito as Lala's lover or still pursues Lala off-scene). While his relationship with Rito received little development, it would seem that Ren has matured enough to put his competition and animosity behind him, and no longer gives Rito a hard time. Though, needless to say, he still cares enough for Lala that he's still reluctant to acknowledge Rito as a fitting suitor for her.

Ren and Rito don't seem to be at odds anymore and may have established a better relationship. However Ren reminded Rito that Lala is a very kind and caring person and questioned him if he is taking advantage of her kindness, which Rito thinks deep down that it might be true. Including Run, Ren advices him not to do something to make girls cry.

Run Elise Jewelria
Run is Ren's female counterpart, who's shared the same body with him since birth, until they reached "adulthood", and the two to permanently split bodies. Ren hates R ito but Run is deeply in love with him. It was through him that she experienced the kiss he accidentally had with Rito, the cause of her love for Rito. Run finds Ren pitiful for picking a fight with Rito over Lala.

It was seen that Ren and Run are capable of communicating with each other while sharing a single body. They generally took turns with who lived in the shared body, while the other slept. Run eventually became the more prominent personality, and she would sometimes turn back into Ren in situations that are very undesirable for him, sometimes even intentionally when the situation is undesirable for her.

After they became separate people, they began living together. It's not known in detail what their lives are like together, or how they get along. Ren has shown a slight care for Run, like when he advices Rito to not hurt her or any other girl.

Kyouko Kirisaki
Ren doesn't seem as close to Kyouko as Run is, but s he is aware of their ability to swap mind and gender through a sneeze. She has helped them from changing when she and Run are performing together, but this is no longer necessary as the two have separate bodies now.

While Kyouko and Ren are likely to have been acquainted to each other through Run, they have never been seen together before.

Trivia

 * Ren is physically similar to Hatsuharu Sohma in Fruits Basket.