You and Castiel were both angels except you had recently died and went to heaven but Castiel on the other had had been their probably for 600,000 years or more but that's just a guessed you had made.

Since you are a newer angel you didn't quite have a real rank just called angel. Castiel had the rank of Powers and that's the rank you actually wanted to get which sooner or later after alot of begging to God he finally gave you a apprenticeship with Castiel.

At first it was honestly hell training with him, he was just straight up ruthless training and fighting with you even though you basically kinda just started but after a couple of months of training you were almost up to his level and were able to dodge most of his attacks. Because of that he would then take you on missions with him patrolling heaven and eventually going down to earth and killing off demons. You both soon got a mission to go to a church taken over by a bunch of demons on earth.

Castiel had just finished stabbing a demon in the chest and you cutting off ones head "This should be the last of them." He said wiping off some blood from his cheek while looking around for anymore in the shadow.