Kinji obviously means a lot to Reki, being that he was the first person to instill human emotions within her. Later on, Reki has shown to be quite attached and attracted to Kinji, and this is most shown in the events of Volume 9, Chapter 4. Reki is also impossibly loyal to Kinji, as since her culture has called for such drastic actions, she was willing to die for him in the tradition of the "Last Bullet". She never acts flustered around him, even when something happens involving her that may initiate Hysteria Mode, even though she knows about it, although this may be attributed to her general lack of emotions.
The events of Volume 6 Chapter 1 showed that she is fiercely loyal to Kinji, which is made obvious by not willing to relinquish Kinji. Combining the male and female tendencies of her culture, she had perfectly forced Kinji into a "marriage" and "relationship" and afterwards successfully followed his every order from then onward. However, during the events of Volume 10 after being attack by Kaname, she would turn against Kinji for a brief period of time after he was ordered to make peace with the enemy until after the Lumberjack duel in Volume 11, where the Baskerville girls got their revenge.
In Volume 12, it was revealed Reki had also filed a transfer application in order for her to follow Kinji due to her eternal loyalty, which included living and dying with Kinji. She would also go as far as challenging the Headmaster in order for Kinji to get a proper transfer at the end of March, but stopped after being ordered by him. Reki would also ask Kinji to give her a last name, which ended up being "Yada" after he refused to do it. Reki would reside at Kinji grandparents house during this time period.
Reki is the only other person who can fit the qualifications of Aria, and as such, they are somewhat more friendly than Reki is to others. Aria also views her as a good friend, for that sole reason. They are the only people, and both girls at that, in Butei High that have an S-Rank.
However, Reki and Aria are both rivals in terms of Kinji's affections, and Reki's somewhat recent moves on Kinji had emotionally disturbed Aria to an amazing point. They are stubborn when it comes to this matter, as shown in the events of Volume 7, Chapter 3. They eventually did patch up their friendship following the Cao Cao Triplets case.
Haimaki, to Reki, is as loyal as Reki is to Kinji. Haimaki risked his life multiple times. When Juujuu's Shar-pei were closing in on Kinji and a wounded Reki, Haimaki valiantly fought off the entire pack and bought enough time for his master and Kinji to escape despite suffering grievous wounds. He managed to survive, much to the admiration of Reki and Kinji.
Reki shows gratitude towards Haimaki in her very slight expression.
Reki shown to be jealous of Moe relationship with Kinji and see's her as another rival for his affection. Reki usually gives her sullen glances and gets angry at Kinji whenever he mentions her name.
Reki seems to have a good relationship with Tamamo based on their limited interactions in how Tamamo let Reki wash her. However, Reki was jealous when she saw Tamamo sleeping with Kinji.