Who was Jephthah’s daughter in the Bible

Jephthah daughter became her father’s regret because he had vowed to God that if God grants him Victory against the Ammonites he would sacrifice the first thing that comes out of his house on his return. Never did he expect that his beloved daughter would be the first to come out to greet him.
Despite his regret and thoughts of not keeping to his words, his daughter persuaded him to honour his promise and sacrifice her.

“My father,” she replied, “you have given your word to the LORD. Do to me just as you promised, now that the LORD has avenged you of your enemies, the Ammonites.” JUDGES 11:36

The story of Jephthah and his daughter raises some difficult questions, such as whether Jephthah should have gone ahead with his sacrifice.
Even so, one thing is clear: Jephthah’s daughter must have been a very devoted daughter and very concerned that her father remain true to his word.
Jephthah was a man who had become judge of Israel by leading the Israelites to victory against the Ammonites who were oppressing them.
Jephthah had been driven from his brothers, but the leaders of Israel brought him back in order that he might help them win the battle. As Jephthah was preparing to battle the enemy, he made a rash vow to the Lord by promising to sacrifice the first thing that came out of his house to greet him when he returned home after victory. Though that is a promise we have trouble understanding today, it was a vow he intended to keep.
Jephthah was indeed victorious. To his surprise, his daughter was the first to greet him as he arrived home. Despite Jephthah’s regret, his daughter urged her father to remain true to his vow before the Lord and offer her as a sacrifice. The only thing she requested was some time to go away into the hills with her friends for two months to mourn that she would die before ever marrying (Judges 11).
Apparently the fate of Jephthah’s daughter left such an impression on the people of Israel that they initiated a custom in which the young women would go out for four days to commemorate her death (Judges 11:39–40).