I still haven't watched SAO War for Underworld, but I liked Deku's development throughout the four seasons of MHA. Kirito is a very plain character so far.

Kirito has far more character development. Deku is the one who is quite uninteresting.

I repeat I only watched SAO I, II and Alicization, but so far he's not very interesting to me. I admit Deku develops VERY slowly, but I loved his personality in season 4.