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.

Personality is not character development and Kirito goes through more character dev in S1 & 2 than Deku will probably ever go through. You prob just don't know what char dev means.

"In fiction writing, character development is the process of building a unique, three-dimensional character with depth, personality, and clear motivations. Character development can also refer to the changes a character undergoes throughout a story as a result of their actions and experiences."

As I said, how Deku changed his personality and motivations from season 1 to season 4 is what makes him more appealing to me.

Ez win for Kirito