The main antagonist's name is Scarlett Mantis with two Ts in her first name, not Scarlet Mantis. This can be seen as she introduces herself at Stage 1's boss fight on Normal difficulty. Here's a video that shows it:

https://www.youtube.com/watch?v=JseemwPG2xU&t=167s

Despite this, The Scarlet Heart boss still has only one T in "scarlet", probably because the word is also the name of a shade of red, and the tank is red also.

Hopefully everyone will see this and help adding the missing letter T to her name on this wiki as well as in the future.