Here is the full list of all 21 Wild Journey achievements.
Romance Megumi
Fall in love with Megumi
User your charm on Ruri
Romance Ruri
Fall in love with Ruri
Use your charm on Aiko
Romance Aiko
Fall in love with Aiko
Use your charm on Sayo
Romance Sayo
Fall in love with Sayo
Use your charm on Shizuku
Romance Shizuku
Fall in love with Shizuku
Use your charm on Koyuki
Romance Koyuki
Fall in love with Koyuki
Use your charm on Fuyu
Romance Fuyu
Fall in love with Fuyu
Use your charm on Megumi