After defeating Regalla, you can either kill her or spare her life. The third option does nothing.
If you kill Regalls, then you will get her unique bow. This is a very powerful weapon, so I recommend choosing this option.
In this case, Regalla will keep you company in the last battle. You won't get her unique bow, but you will avoid an extra fight with Ghosts.
The developers of Mobile Legends: Bang Bang are preparing a large-scale update called “Breaking the Waves”, which will be available on March 19. Players can expect a new hero, new events, rewards, as well as changes to the seasonal system.