Yami

Golden Darkness (Konjiki no Yami) is a major character in the To Love-Ru anime and manga. She is a lonely assassin who is initially hired by Lacospo, one of Lala's arranged suitors, to kill Rito. As she spends more time on Earth, she builds a very strong relationship with Mikan Yuuki, Rito's sister, and maybe even Rito himself.

Mikan Yuuki
Yami's most significant friendship is with Rito's younger sister, Mikan Yuuki. This may be because of Mikan's frequent friendly advances on Yami, and Mikan constantly talking with and helping Yami. In Chapter 139, her and Mikan switch bodies for a while, both wanting to see what the other's life was like. Mikan is the only person Yami actually calls "friend" and is seen by her side quite a lot, and vice versa.