1. He's eleven when he attends his cousin Obito's memorial service and notices the pretty girl struggling to not cry. He smiles at her and she returns it weakly.
2. She's thirteen when she sees the curly-haired Uchiha at Obito's memorial service. Even then, she's not sure who she's looking at, especially when he grins at her.
3. The revelation of her transplant of a sharingan to a non-clan member that causes such fury in the clan marks the beginning of his fascination with her and he hangs around the edge of her meetings with Fugaku-sama.
4. They don't meet again until the funeral of the Fourth Hokage but she doesn't really remember anything except Kakashi's blank face from that day.
5. At thirteen and a half, he ends up in her care after either a training accident or a mission gone wrong, she never gets the full details but his injuries look suspiciously like cat scratches. Although he's known her name for a while, it's only then she learns his.
6. When she turns sixteen, he leaves flowers outside her apartment door.
7. She doesn't really notice him until he's a fifteen-year-old chuunin and soon to be jounin.
8. However, he has always been very aware of her. One thing Itachi recalls from his childhood is Shisui tending to abandon his promises of training in favour of hanging around the hospital.
9. They get together on a cold winter day. Her face, flushed and smiling, as he asks for a date is something that he never forgets.
10. There are certain things that they do not discuss. One is Obito. The other is his doujutsu.
11. He's constantly surprised that he's the first person she's dated (until a wolf-masked ANBU takes him aside for a talk, anyway).
12. She's never seen him fight and doesn't particularly want to either. It's bad enough that he ends up in her care after missions more often than not. (She suspects that he exaggerates his injuries most of the time to see her though.)
13. He knows perfectly well what his clan's opinion of her is and equally, they know that repeating it in front of him is a very bad idea.
14. She finds Itachi unsettlingly intense while he considers her brother creepily polite.
15. He likes giving her things, trinkets which he spots on missions to far-flung places. She thinks he spoils her but he can't help thinking that she deserves a lot more.
16. The future is something they never really talk about despite there being increasing pressure on him to settle down. He refuses mostly because he knows whoever they'd pick wouldn't be her.
17. In the weeks before his death, she notices a growing tension in him that he tries to deny.
18. There's a note slipped under Itachi's door. I believe you. The writing is slightly messy. (He'd always teased her about her tendency to slip into a medic's scrawl unconsciously.)
19. She takes her first mission in years shortly afterwards. Kakashi tries to prevent her by directly appealing to the Hokage but she's already gone.
20. There is no funeral for either of them. His body disappears and hers is never found.
This was inspired by In Darkness Is Light's amazing story, Lucky Charms.
Rin's fate is left ambiguous on purpose so you can read it any way you want. (It probably wasn't suicide though.)