The story is about Maria Enders, she is now middle aged and a director wants to revived her character that she did 20 years ago. This time director wants to cast Maria as another character in the new version. Clouds of Sils Maria is about Maria's fighting to adjust her mentality for the new role and story about her thoughts, beliefs and re-adjustment with young thinking. At least that's what I figured.
Kristen / Valentine is Maria'a assistant, also a friend may be. She advised Maria in several topics, comfort her in all odd situations. She also helps her to built the character within Maria that she will do. Rehearsal partner.
During the adjustment with the new role of Maria, Maria expressed her point of view of her character. But Valentine disagreed. She keep telling her about her interpretation which had some effect on Maria. Valentine realized that and wanted to leave her / at least didn't want to do the rehearsal.
One day they were on a hike together to watch snake(a cloud formation in between mountains) They again started arguing about the character. At the end Valentine got upset and start walking. Then after one scene there is no Valentine. After a while when Maria realized she isn't around, Maria started shouting. But there was no Valentine. Disappeared in the mountain.
Next is very strange. We saw a new assistant. There is no further mention of Valentine in rest of the movie. Awkward. Where did she go! Is she dead? Did she has an accident? Did she left because of the argument? Where did she go? And why? What did I miss? What did it mean?