The stitches were originally part of the back story of Ruben the scarecrow - before becoming a monster, he was mutilated by some gang who also murdered his girl friend.
Now... this version on the other hand...

Illusionist is technically the same guy, but just different version of him. Illusionist is also the most human looking Anima in Vas Diaboli's crew... up until when and if he removes the stitches from his cheeks, you'll see his mouth is unnaturally large.