Quinn is Alex's best friend and former neighbor from his childhood. Since Alex was an only child, he grew up with Quinn and developed a crush on her overtime. Quinn cares for Alex and has addressed many times in her life. When he and Quinn were kids, Alex would often spend the night with Quinn and her parents. After his sixth birthday, Alex and Quinn found Abraham and Cindy murdered which horrified and angered Alex. After Abraham and Cindy's funeral, Alex moved in with Quinn while he was awaiting for his adoption by the Fisher family. Following his adoption, Alex was separated from Quinn, upsetting Quinn who realizes she has feelings for Alex. One night on Alex's thirteenth birthday Quinn spent the night with Alex and his adoptive family, but she fell asleep in Alex's arms after watching Friday the 13th. In the Pilot, Quinn gets very jealous of her best friend's relationship with Rachel Berry, but she promised him she'd not get involved because she cares too much about him to wreck their friendship and her feelings for him grew stronger. When Alex finally joins New Directions, Quinn decided she'd join too, just so she can be more closer to Alex then usual and to spy on Finn. When Sue suspected, Alex was distracting, Quinn, Santana and Brittany from taking down the Glee club, she'd force Quinn into lying to him, but Quinn refused lying to Alex. Eventually, Quinn tells Alex she's pregnant, but the baby belonged to Puck and not he or Finn, Alex was angry and he wanted to kill Puck, but Quinn made him promise not to. However, he broke that promise when his anger got the best of him. Quinn questions Alex's actions, which makes him reveal that he's in love with her, before he walks out the room in rage.
Rachel, you're a butt.
(Best friend; ex-girlfriend)
Rachel was one of Alex's closest friends from his childhood and was his first girlfriend. In the episode Acafellas, Alex and Rachel call their relationship off due to having feelings for other people Quinn (Alex) and Finn (Rachel). After their break up they remained friends, but also seem closer then before. Despite breaking up with her, Alex still cares very much about Rachel, as when he got his revenge on Jesse St. James for egging her and for also making fun of his parents murder
You're my best friend Santana, you'd make a great godmother to my child one day.
(ex-girlfriend; best friend)
Although it's never explained how long Alex and Santana have been friends, but it seems they've been good friends for a few years. In Showmance, Santana started to display feelings for Alex, as evidence by the way she looks at him during class and rehearsing songs. While at the hospital after Alex had been shot, Santana lied next to Alex in his hospital bed, vowing to be there for her friend and classmate as he's recovering. Later, after breaking up with Rachel, Alex begins to date Santana.
After Alex was shot, Brittany became very fearful for him seeing a classmate she hardly knows, in a coma. Following the incident, Alex and Brittany become very close friends, acting like brother and sister.