Renji goes by the pseudonym "Miya" for most of the game. He is very friendly to his customers. His real name is only revealed on Natsume's path where he is revealed to be her father.
Renji used to work alongside Hisae on several large-scale alchemy projects. One of their bigger projects was to create a copy of the Noblemen's Stone. Natsume disagreed with it and disliked Hisae for initiating it. Natsume swallowed the incomplete Stone and fell into a comatose state and her body disappeared. Renji was unable to find her body and he blamed himself for not being able to protect her.
Renji runs a variety shop in downtown Eiden Island. He sells everyday items as well as alchemy implements. His shop was visited several times in the game. Kazushi attempted to make conversation with Renji but he refused to talk about himself.
On Natsume's path, with her hints, Kazushi deduced that Renji is her father. Renji then revealed Hisae's experiment and Natsume's fate. Kazushi and Renji then searched and found Natsume in an underground cave. Kazushi entered her dream and woke her up and she happily reunited with her father.
Natsume and Renji then explained the process of alchemy. There exists an ideal world where a copy of everything in the real world exists in a perfect state. Alchemy is the process of trading an item from the real world with that from the ideal world. When Natsume swallowed the stone a few years ago, her spirit was trapped in the ideal world. Kazushi has a special ability to freely travel between both worlds.
Renji then chose to work at the school's shop to stay closer to Natsume. He brought along a katana that signified his fatherly protective love.
Some time later, the Hisae decided to act to retrieve the Noblemen's Stone from within Natsume. She conducted a large-scale experiment to sacrifice everyone in the school to complete the stone. Renji confronted Hisae and talked her out of it. They then discover that Kazushi and Natsume was trapped in the ideal world.
Hisae suggested destroying the ideal world's island's Noblemen's Stone to return them to the real world. She knew that that will cause the real world's Noblemen's Stone to disappear as well but she explained that she had moved on.
On Mizuki's path, Renji aided the island's doctor using medicinal alchemy. He diagnosed Mizuki's legs and concluded that restoring her legs would be difficult even with alchemy.
On Mei's path, Hisae and Renji revealed that Mei is an homunculus created by them. They explain that she can continue living if she ended her relationship with Kazushi. Renji offered to wipe her memory clean.