It could be a group of fallen Jedi. They turned to the dark side for various reasons, like power, revenge or disillusionment. As they were once Jedi, they know the temple's layout and weaknesses well, making them very dangerous villains in the attack.
In a Star Wars Jedi Temple Attack fanfiction, perhaps there's a traitor within the Jedi. This person helps the attackers, believing the Jedi have become too corrupt. The attack could lead to a great battle where Jedi have to fight not only the external foes but also deal with the betrayal from within.
Perhaps an Imperial Inquisitor who has some special powers and a vendetta against Cal. They are known for hunting Jedi and would make a great antagonist. They could be leading a group of stormtroopers specifically trained to deal with Force - users.
Darth Vader could be a great main villain. His dark power and iconic status in Star Wars would make him a formidable foe for Supergirl. He could see her as a threat to the Empire's rule and try to eliminate her. His mastery of the Force and lightsaber skills would put Supergirl in a tough fight, especially if she's not familiar with the ways of the Force at first.
Obviously, Luke Skywalker could still be a main character. He has a lot of potential for further development as he continues to grow as a Jedi Master. Leia could also be central, as she takes on more leadership roles in the new galactic order. And of course, there's always the possibility of new characters being introduced, like a young Force - sensitive alien who becomes an apprentice to Luke.
It could be a new character created specifically for this story. Maybe a young Jedi who starts to question the Jedi ways and gradually becomes a Grey Jedi.
It could be a new Sith Lord who is trying to finish what the previous Sith started. This villain might see the child as a threat to their plans of taking over the galaxy.
Ezra Bridger could be a potential Grey Jedi. He already has shown a willingness to bend the rules a bit in his use of the Force. His journey from a young, inexperienced Force - user to someone more complex could lead him towards the Grey Jedi path as he sees the limitations of both the Jedi and Sith ways.
Luke Skywalker is a main character. He is the son of Darth Vader and Padmé Amidala. He became a powerful Jedi and played a crucial role in the defeat of the Galactic Empire.
Luke Skywalker is a main character. He is a Jedi who plays a crucial role in the fight against the Empire. Another is Obi - Wan Kenobi, an experienced Jedi who mentors Luke. Yoda, the small but very wise Jedi Master, is also important. Anakin Skywalker, before he turns into Darth Vader, is a significant Jedi character as well.
Luke Skywalker is often a main character in many of these novels. He is a key figure in the Jedi Order's revival. Another important character is Rey, who continues the Jedi legacy in the new era. And of course, Obi - Wan Kenobi features in several novels as well, especially those set in the pre -quel era.