Their relationship in the fanfiction is likely to be full of mutual understanding. Since they are getting married, they must have a deep connection. Takato might be the more gentle and patient one, while Rika could be the strong - willed partner. They balance each other out.
I'm not sure specifically which 'rika' this refers to. It could be about a person named Rika. Maybe it's her personal experiences, her growth journey, or something related to an event she was involved in.
It's hard to give an exact time as it depends on the manga's plot development. But typically, Yuta Okkotsu shows up when certain events and character arcs lead to his introduction.