You are able to change the names of your characters by editing the game's text files. Open your Rogue Legacy folder, open the 'Content' folder, and you should see HeroNames.txt and HeroineNames.txt. Add as many new names as you like, but bear in mind they must be no longer than 8 characters.
If you are struggling on the fourth boss, try this glitch for an easy way to win.
First, get an Archmage with the Time stop spell. Then, hit the big blob until it has very low health, but don't let it split. Stop time once you are at a point where you can stand next to him on a platform and keep using your basic attack. You should be able to keep this up until you destroy the boss, providing you have enough damage. You can use the minions spawning out of him to get Mana back to prolong the spell.