Aria's opinions often clash with Kinji's, which is why she nicknames him "Stupid Kinji." Aria, however, is in love with him, but she doesn't show it due to her prideful nature. An example is when Kinji got sick, and Aria anonymously took the trouble to buy him medicine from a far location.
Aria had initially fallen in love with Kinji not too long after they first met, but it was his Hysteria Mode that she found highly attractive. But later on in the series, it is constantly shown she actually harbors romantic feelings for Kinji and is too scared to confess to him. Instead, she keeps Kinji in a locked-relationship and doesn't allow Kinji to move in on other girls and vice-versa.She can be very clingy and jealous if he even talks to other girls, especially Riko or Shirayuki. She became particularly jealous of Reki when she forcefully claimed Kinji as her husband and went as far as ending her friendship with Reki.
During the time period where Kinji had secretly transferred out of Butei High, it was shown that she really missed him and life was not the same without him. However, after she rescued Kinji at the Katagaka Mansion, she was happy to see him again and was waiting for him when he came back to Butei High.
Via further insight, her relationship with Kinji is almost not unlike Holmes portrayed through the original novels towards his close friend John Watson and a few others, in that Aria tends to annoy Kinji because of her immature habits and causes him much irritation; though there are times Kinji and Aria trust each other more often as the series progresses, and as Kinji finds himself as the Watson to Aria's Holmes, more used to her absurd antics everyday, but working better with her at the same time regardless.
Initially their relationship got off to a rocky start after Riko, who misinterpreted why Aria had Kinji's belt, proposed the idea that Aria and Kinji were lovers to their class. As the series progressed, Aria's disdain for Riko only worsened when she learned that Riko was in-fact the Butei Killer. Despite learning of Riko's heritage and a rivalry that dates back to the 19th century, Aria isn't as concerned with defeating the descendant of Lupin as Riko is with beating the Holmes. However when Riko returns after being kicked out of IU Aria slowly begins putting more trust into Riko as she not only offered to testify for her mother but also help pin down Vlad. Eventually,she allows her to join Team Baskerville. Riko does love to tease Aria, often times flaunting her significantly larger bust and openness to get close to Kinji.
Aria considers Reki somewhat like a friend, even though the two don't necessarily socialize. This is due to both having similar traits: both being S-Rank and are short, petite girls. Aria also respects Reki, and it should be vice-versa.
However, Aria may see Reki as an extremely powerful rival in the war over Kinji's affections, and the events shown in Light Novel Volume 6 have shown that Aria was willing to break off their "friendship" over Reki's actions. However, the two now see each other as equal respectable rivals.
Aria and Shirayuki originally had a rough relationship due to Shirayuki's morbid obsession for Kinji. Aria would always have to keep her guard up in the event Shirayuki snaps assaults her. Following the Durandal case, however, their relationship progressively got better and Aria "recruited" Shirayuki into her team.
Aria is Kanae's daughter, and Aria loves her very much, to the point where she'd be willing to put her life on the line to prove she's innocent.
But like most mothers, Kanae has the capacity to embarrass Aria in a tight situation (Episode 4).
Aria is initally shown to dislike him/her for having a connection to Kinji.
Aria is jealous of Kinji's relationship with Moe as shown when she rescued her along with Kinji and Kikuyo at the Katagaka Mansion, and found her hugging onto him and thinking she was his normal life girlfriend.
When Aria saved Kinji, Moe, and Kikuyo at the Katagaka Mansion, she was shown to be jealous of Kikuyo, especially when she saw her hugging onto Kinji.