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 and show his admiration of her by complementing her beauty. Due to his lack of appearance, Lala seems to easily forget him.
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 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 Rito 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 advises Rito to not hurt her or any other girl. Technically the two are now twins.
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.