Before and After Kari
I think that the relationship between Tai and Sora changed after Kari arrived in the DigiDestined group. I will show you some examples and you can agree with me or not.
Before:
In the beginning Tai and Sora were the only two who were already good friends (besides Matt and T.K. who are brothers). They played in the same soccer team and they were not just regular playmates, they were friends (as we see in one of Sora's flashbacks). They knew other DigiDestined but they didn't seem to be very good friends. In dubbed first episode Tai only says something nice about Sora (that she is ok for a girl). If they were already good friends, he probably didn't say stuff like "Joe would wet his pants if you scared him" (or would he o.o?). So it was kinda reasonable for Tai and Sora to be together most of the time.
They chatted a lot and got along well, while Tai was arguing with Matt or Joe. Sora on the other hand was nice to everyone but she was still somewhere around Tai most of the time. Like when they came to Pagumons' village and she was worried and he calmed her down when Meramon attacked Poyomons' village they were the ones who lead Poyomons in the abandoned ship when they talked about Sora's worries at the pyramids when they went to find Joe ... You can see that there are many proofs that they were good friends. They could have done these things with someone else but they didn't.
And not just being together all the time, they were worried about each other. When Sora was kidnapped by Datamon, Tai cried because he was too afraid to save her. On the next day he was the one who saved her and risked his life for her. And when he disappeared in the real world, Sora was the one who cried: "Come back, Tai!" She was also the one who left the group first to go find him. When he finally came back to the Digiworld, Sora was hiding from them because she thought she didn't deserve her crest of love. When they found her, Tai was the one who said that love was the perfect crest for her and tried to calm her down when she started to cry. Matt for example seemed like he didn't really care about her feelings.
Almost every Taiora evidence is from the time when Kari wasn't the DigiDestined yet.
After:
Kari became the 8th DigiDestined in episode 34. Her Digimon partner Gatomon was still part of Myotismon's group and that meant danger for Kari because Myotismon wanted to find the 8th DigiDestined and kill them. He was searching for children who could be that person. Gatomon was kidnapped but she didn't tell her boss who her partner was. Tai went to get Gatomon back and he left Kari with Sora and Matt to protect her. That is the beginning of Tai's grave concerns for Kari. When they came back in the digiworld Tai spent most of his time somewhere around his little sister. For example, when they were trapped by Scorpiomon in a house full of sand, Tai was only protecting Kari (and T.K.), Sora wasn't even near him.
If we go back to a time before Kari, we can remember that episode where Tai was protecting Sora from Mammothmon with his body. Suddenly Kari was ill and Tai and Izzy went to find a cure for her. Sora, T.K., and Kari were hiding in a random house and this house exploded when Tai and Izzy finally came back with the cure. Tai was only yelling Kari's name, even though Sora and T.K. were also in the house. But of course, they were alright. At the end of this episode they all fall in a giant hole and Tai was again yelling Kari's not Sora's name. They were parted from each other, Tai and Izzy in one group and Sora, T.K., and Kari in the other. The whole episode Tai was trying to get to Kari (not to Sora or T.K.) and in the end when they meet again he is really happy to see his little sister and he takes her in his arms, ignoring Sora.
There is only one little Taiora evidence from the time after Kari's arrival. It's Sora, who says that she needs to go to the soccer training because Tai will be there.
In the second season Tai doesn't look after Kari that much and he goes back to Sora but it's already too late. She is with Matt then.
