Celine

Celine is Lala 's gift for Rito's 16th birthday. She is sort of a child-like humanoid/plant alien, but originally was a rare multi-story, sentient sunflower kept in the Yuuki's residence backyard.

Appearance
Originally, she was an extremely huge sentient sunflower with purple petals around the face, which is just a mouth, she also has vines to be used as hands. Now she resembles a human child with blue eyes and light green hair with a flower on top with light pink petals. In the anime, she  only  has two long toe's on each of her feet. In the manga and To Love Ru Darkness, her feet appear more like those of human babies.

In the bangaihen of the To Love-Ru Darkness Manga Series, Celine became a young woman with the effect of the moonlight in a full moon. As seen at the last page of the latest bangaihen series, the effect only last for the night of the full moon.

Personality
Celine is often seen to be running around really happy and playfully. On some occasions she seems to quite curious, as seen when she plays with some of Lala's inventions or Momo's plants when she has no idea what they do, which may cause a lot of trouble like when  she was somehow able to use a cellphone to take a picture of Lala's naked body and emailed it to Yui and she managed to use Lala's remote massager to mercilessly grope and molest Tearju via the device's touchscreen.

As an infant, Celine has a tendency of wanting breast feeding, so she constantly jumps on to other women's breasts and and begins sucking them. Celine enjoys playing video games, watching the Magical Kyouko   show, gardening and eating ramen. Celine is normally very well behaved, albeit troublesome in her young age, but can become fussy and impatient just like a human toddler when she is held back or comes across events that annoy her. She has a vast appetite, due to Lala experimenting on feeding her different things, and even enjoys eating Yami's Taiyaki Soup when she makes it.

In her grown up body, she seems more calm and mature but still slightly playfully. She reffers to Yuki and Mikan as her family and desires to be close to them.

Plot
Although seemingly monstrous, Rito cares for Celine deeply, referring to her as his family. The plant behaves akin to a human; perspiring when the climate is humid, wearing an oversized scarf during a chilly day and eating a bowl of ramen when hungry (though Celine still needs the basic nutrients of water and sunlight). When Celine seemed to have contracted a deadly disease, Rito was fully dedicated to find the Luckberry to save her from the Kare Kare disease. With his friends, they journey to the S-level-danger class planet Misletoe to retrieve the antidote, only to return and find the plant all dried up. In reality, it was germinating and from its single seed, burst out Celine; taking the appearance of an infant girl, with a flower on her head. This event even shocked Momo, who is very knowledgeable with the galaxy's flora, stating Celine's biological makeup is still a complete mystery to her.

A running gag as a child is Celine will playfully tackle any girl and attempt to suck milk from their breasts, or jump onto someone out of nowhere, most of the time causing the incident for the chapter's story. Comically mistaken for being Rito's daughter, Celine makes guest star appearances in the OVA adaptation (as a plant near the end of OVA 4 and as an infant in OVA 6 part 3) and becomes a regular character with her infant form in the new anime series.

Powers and Abilities
Being originally a plant, she has a connection with other plants like Rito and Momo do, so it's possible that like Momo, she can also communicate with plants. Even though she talks in a somewhat baby-like language, her child form can only say the word "Mau". However Momo is still able to understand her. Celine may be able to understand what other people say and seems smart enough to be able to read as she is sometimes seen with a book.

Because of the   parental love   Rito  had shown her, whenever she gets drunk (off cola), she playfully walks around spraying pollen out of the flower on her head, causing anyone who breathes in this pollen to grow a tiny flower on their head and temporarily fall in love with Rito. The effects wear off after 30-45 minutes or so, and the flower falls off of their head, suffering no memory of the events of being sprayed or the events that happened when under the pollen's influence.

Despite her small size, she is capable of jumping really high, able to reach a person's face, or maybe even higher.

When the moon is full at night, Celine transforms into an older version of herself and is even able to speak. When the night has past, she turns back into her infant form.

Rito Yuuki
Ever since Lala gave her as a gift to Rito on his 16th birthday, he has been taking good care of her as if she was part of his family. This great deal of care for Celine was shown when was willing to go up against impossible and dangerous odds to retrieve a cure for her. She see's Rito as her parent because of how he took care of her and because of her child-like appearance and her closeness with him she is often mistaken as his daughter, like the time when Rito's mother thought he was her dad she immediately asked who was the mother. She is one of the few characters with unique traits that causes trouble for him. She loves him deeply, enjoying being with him, getting to see him and getting cared for by him.

Momo Velia Deviluke
Celine and Momo share a sisterly bond, communicating and socializing with each other via Momo's inborn ability to speak with plants. Celine is often taught right from wrong by Momo, being told what she isn't allowed to play with or touch after doing so, to which she seemingly understands and feels guilty about. When given the chance, Momo would use Celine's pollen to advance her harem plan.

Mikan Yuuki
Mikan and Celine get along really well and they are seen playing together on some occasions. Sometimes Celine would copy Mikan's motherly personality. Mikan was the one who named her. Mikan can act motherly towards Celine especially after seeing her "all grown up".

Yui Kotegawa
Celine has shown a possibly strong liking to Yui, as pointed out by Rito when she hugged her. She was once mistaken for being Celine's mother and in a way as Rito's wife, to which she greatly denies, but because of this Yui would sometimes have day dreams about the three of them as a family.

Trivia

 * She and Mikan are customers at the Stray Cat in the manga series Mayoi Neko Overrun! in Chapter 1 Page 37