See complete translation in example sentences

In English, we don't really use the work "character" the way it is used in Russian.

Character is used when talking about a role played by and actor: For instance a character in a play.
We also use characteristics when talking about attributes of a person.
You could also use character in a sentence like: "He was a man of character" which suggests a aura of formality, dignity and completeness.

Finally you could also say "character traits". That might make sense here, but honestly I think that simply personality works better.

As for mannerisms, I considered swapping that word for behaviors. But mannerisms implies more than just how someone behaves, it also includes how they dress, how they walk, or other routine habits. So, I decided to stick with mannerism. Also, the "all" is implied here. If she was only making some mannerisms boyish she would have to state that it is only some, otherwise it is assumed that she is making all of her mannerisms more boyish.

Finally <boyish>. Often in English you can throw an ish or ly on the end of a work and it will imply "approximately like a boy". In this instance, she can't really be a boy but is trying to be more boyish.

Plus, I think boyish is just a fun word.

Good luck! Let me know if you need anything else!

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Note added at 5 hrs (2018-07-04 15:36:26 GMT)
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As per El oso’s comment. There are no context clues to determine whether or not this is first or third person. So another possible translation would be, “She tried to make her personality and mannerisms more boyish.”