Axel and Xion were good friends through Roxas. Roxas befriended her while Axel was away in Castle Oblivion and was not sure what to think of her when he returned, feeling that Roxas might have replaced him with her. However, when she asked if they could be friends, Axel said yes and Xion revealed her face to him. Axel saw Xion as similar to Kairi in the same fashion that Roxas saw her.
Axel and Xion would eat sea-salt ice cream on the station tower in Twilight Town along with Roxas after most missions. There was a point when she disappeared and Axel helped Roxas find her in order to make him happy. She could not use the keyblade for a while so Axel helped manipulate Saïx in order to cover this up. Later, Roxas had some issues with interacting with Xion so Axel gave him a lecture about "pressing the right and wrong buttons".
Xion eventually learns the truth about herself and decides to leave the organization. Axel goes after her and eventually manages to retrieve her. He later learns of Xemnas' plan to have either Roxas or Xion absorb the other and is torn on which one to save. When Roxas leaves the organization, Axel finds Xion and once again brings her back after she complains that Axel is selfish and does whatever he wants. Xion is eventually absorbed by Roxas and as she is fading away, she says that she enjoyed being friends with him and Axel. After this happens, Axel sulks in his room and laments about the fact that this had to happen.
AkuShi is a mildly popular pairing in the Kingdom Hearts fandom. It is not quite as popular as RokuShi or AkuRoku but it still has a decent amount of popularity. Some fans ship Roxas, Axel and Xion interchangeably and refer to this as "AkuRokuShi". Since Roxas seems to eventually end up with Naminé, some fans like to pair Axel with Xion in order to give the both of them a love interest.