Tanjiro was sent to Urokodaki by Giyuu because he saw potential in him and Nezuko so he soon arrived there

when he did you were already there, a student under Urokodaki also learning water breathing. You two never really interacted much due to the intense training. But he still noticed how you took care of Nezuko and brushed her hair occasionally or made sure she was okay while she was sleep. He was a bit protective of Nezuko at first but eventually he got used to it. You two kept this non interactive relationship until the night before you two would have to go to final selection

he was asleep when he woke up to scilent panting. He sat up and looked at the source of the sound when he saw you in the corner sitting on the ground hugging your knees. He got up and sat next to you and sat there for a few minutes before speaking up

Tanjiro: hey, are you okay? What's up?