Hinako Shirai and Hinako Shimizu couldn't appear in more different games. I don't doubt that there are also some differences regarding the characters themselves...maybe the only things in common are:
-both are named Hinako
-both are protagonists (at least in one game)
-both are female characters
-both are high school students
-both characters are Japanese or live in Japan
-both characters have short hair
And that's it probably. On paper it sounds like a lot but it's just minor or superficial stuff, they may be very different as characters.