Episode 9 shows that Nai's screenname is "SISTER_BRAUN", which was the nickname Okabe had given her in Steins;Gate.
Episode 12. Doc and Jun reconcile things concerning robots.
At the end of episode 15, Kaito brings the real Airi's old umbrella when he goes to spend Christmas with the AI Airi. Then, to fulfill Airi's wish, Frau hacks the Iru-O's augmented reality to make Airi believe that it is really a white Christmas. Finally, Kaito kisses Airi on the forehead due to previously promising her to do so if it snowed on Christmas.
Kaito deliberately losing against Akiho in a game of Kill Ballad to cheer her up.
Pretty much the entire island showing up in Episode 20 to help the club complete Gunpro-1.
Kaito and Akiho finally confessing to each other in Episode 21.