The Marine Queen

The very first optional superboss in the game, she hits hard and fast. She only has 10,000 HP though, so it is possible to defeat her if you come prepared. Her trusty diamond floating above her head is able to strengthen and heal her, and has an extremely high defense. She can also OHKO your entire team if you're unlucky; otherwise, she will try to use Time Fissure to send one person on your team outside of spacetime, simply punch out a party member's lights, or seduce a teammate and get them to do her dirty work. The Marine Queen is the only enemy in the entire game that will not give you a game over if you are defeated. Instead, she says "I win <3" and sends you on your merry way.