Is a fiancée entitled to ask for separation from her fiancé due to his unsuitability and bad behavior, even though she performed Istikhara and did not see anything?

If the engagement was merely a promise and no legal contract (aqd shar'i) took place, then the suitor is a stranger to you (ajnabi) and it is not permissible for him to speak with you without a need, or to sit in seclusion (khalwa), or for you to speak to him in a soft and submissive manner. The fact that his family does not visit or inquire about you is not considered a justification for rejecting the engagement. However, if you see that matters will not be stable between you after marriage, then there is no blame upon you for breaking off the engagement, as it is merely a promise that does not necessitate fulfillment. It is advisable to perform Istikhara (prayer for guidance) and seek consultation before making a decision.