At first, Kouji and Takuya aren't the best of friends. Takuya tries to talk to him when he gets his message on his cell phone, realizing Kouji did too. Of course, Kouji doesn't reply back, which makes him seem cold. As Kouji starts to warm up to the group, they become better friends. Kouji jumps and saves Takuya from Duskmon, getting badly injured himself. They still sometimes get into fights on what is right to do next, concerning the bad guys, but they are best of friends.
Izumi and Takuya have an interesting relationship. Sometimes it seems like they get along perfectly fine and then others... well they don't. When they both were looking for Tomoki, Izumi falls and grabs Takuya, and when they land she is hugging him, both embarassed, Izumi slaps him a couple times, and pushes him away. Takuya obviously cares for her, because in the dream he has, he dreams he is laying on Izumi's lap and enjoying it. :3 When they are in the library they get on the topic of him going out with her when they got back to their world, but when she said "what?" Takuya pretended to be asleep.
Junpei is awfully jealous of Takuya, because Izumi gives him more attention then she does to him. He gets angry everytime they are near eachother and tries to butt in. Dispite this fact, they still become friends, especially towards the end, when he realizes that he does in fact have friends. He sometimes tries to help Takuya fight, but he usually gets in over his head ^^;
Tomoki and Takuya have a brother relationship. Tomoki looked up to Takuya from the very beginning when he saved him from falling. Tomoki ends up staying in the Digital World just because he wanted to be strong like Takuya. So, Takuya always makes sure Tomoki is safe, and Tomoki always tries to help his rolemodel. Together they support eachother.