hi so this is my personal opinion but kags differently knew he was in love with shoyo first
Now listen I personally see shoyo as a extrovert and due to being a big brother, someone who is observant when needed and quite emotionally intelligent which helps him become friends with so many people
However due to this emotional intelligence towards other people I think when it comes to himself he may fall behind, not understanding certain feelings he has unlike a friend who is telling him abt how they feel. In my opinion it isn't until the Brazil arc where he faces great introspection towards himself and emotions and feelings both in volleyball and others close & dear to him does he realizes how he feels about kags is different than tsukki or yams or yachi
In contrast kags is seen as a more private, sometimes quiet person, who is definitely more introverted. It is hinted towards that his parents aren't around often if at all and with Miwa being much older than him and likely moved out he spent much time alone training and teaching himself about volleyball and the world.
It is also known that kags is the youngest and in some of his closest friendships or team dynamics people unintentionally hover or go near him, while kags may not understand why they're talking to them he knows to respond and how he wants to respond plus adding in his habit of journaling, actively analyzing his past decisions and thoughts I imagine he knows (even if it annoys & frustrates him) how he feels very well. He might not be able to put direct names on these feelings but he can recognize them better than shoyo
But yea long story short kags knew he was in love with shoyo before shoyo knew he was in love with kags which makes their reunion on court so good bc at that point they both have recognized their love for each other and both are chasing and catching the other simultaneously
(inspired by @kagehiner essays missing them rn 🥲)
