Asking for advice.

I have a character that is crucial to the development of the protagonist BUT at the same time learning to come around from being cynical and depressed and bored.

Here’s what I’ve figured so far:

1. She has principles - she will come out of her cynical shell when she has to but this won’t always be the case.

2. She has goals - obviously, she is willing to compromise BUT not yet in the story that she has to make the do or die decision.

3. She is actually the logic and pragmatism and far more experienced than the protagonist, so although quite young, her depression and sometimes venomous personality is something i focused on to make her bearable and relatable.

What i’m asking for is this:

How do i keep this sustained without teetering towards learning her ‘lesson’ and becoming an optimist or descending further, WITHOUT sacrificing her active participation and efforts?