Why is shisui all of a sudden a relevant character

[Forest]

without KA edotensei would have never been released.

 

[Lightbringer]

Because he's an awesome character that barely had any screen time in the manga

 

[shelke]

No idea why he is important. His Doujutsu was a key factor in the plot though. Which is odd as the author have more depth to the tool than the Sinobi. Oh well, Uchiha techniques have great depth and literary value in this regard anyway.