Yusaku's and Ai's first met when the Hanoi went after Ai planning on destroying the ignis. Ai was blasted to bits however he survives as a eyeball and tries to escape. Yusaku, after realizing that the Dark ignis could be useful to his mission, keeps the Dark Ignis hostage. Ai and Yusaku have a bitter relationship with Ai wanting to have a closer bond,while Yusaku acts deadpan and cold towards him.However during their Encounters with revolver their bond started to grow as they build trust in one another.
Yusaku finally admits how important Ai was to him and that he is his partner. Ai's Creation was revealed to be based off Yusaku just like the other ignis. Yusaku and Ai ultimately managed to defeat Bohman who merged with all the remaining ignis, however this gives Ai lots of Grief and sorrow. Yusaku and Ai want coexistence between Humans and ignis but problems arise each time making things even worse.
When Ai became the antagonist, Yusaku still wanted to believe in him despite his crimes. Yusaku still believed he could save Ai and ask him to go into hiding so he won't die, as Yusaku as admitted he wouldn't like the thought of Ai dying. During Ai's Duel with Pandora Ai states he wants to share his feelings to a special person. Ai and Playmaker/Yusaku dueled in the finale as Ai was defeated,he revealed he just wanted to protect Yusaku from death as In the simulations he saw Yusaku would perish if Ai stayed in society.A nice thing to know is that Yusaku's/Playmaker's Accesscodetalker is a symbolization of his bond with Ai and Dark templar is quite similar to Accesscodetalker.Yusaku states while crying how Ai's name meant love and Ai states that he liked the name Yusaku gave him ,along with stating he knows what it means now while staring at yusaku with a smile.As Ai disappears He states he loves Yusaku as Yusaku goes to the real world and holds Ai's Soltis body,screaming Ai's Name in sadness and sorrow.
With Ai possibly being alive in the internet, Yusaku sets out to find Ai and bring him back.
This ship raised in popularity with their Contrasting Dynamic in comparison to other protagonists. It eventually grew as one of the most popular ships in the VRAINS fandom, especially with Ai's confession and Yusaku's heavily hinted to have reciprocate feelings.
There's even blogs pointing out the romantic undertones/feelings they had for one another within the series.
On AO3, Alballshipping is the most written ship for Ai, and the third most written for Yusaku. It is also the third most written ship in the Yu-Gi-Oh! VRAINS tag.
- Ai is apart of Yusaku in a similar way to Yuma and Astral from Yugioh Zexal.