Near Rhymes (similar sounds, but not perfect):
* Felsing (made-up word, but sounds similar)
* Blessing
* Kessing (made-up word, but sounds similar)
* Messing
Eye Rhymes (spelled similarly but pronounced differently):
* Helsing (if you need a word that looks similar)
Slant Rhymes (somewhat similar sound, but not as close as near rhymes):
* Fencing
* Pulsing
* Sensing
Other options:
* You can also use words that rhyme with parts of "Hellsing", like "singing", "flinging", or "ringing".
* If you're writing a poem or song, you might be able to use a different word altogether that evokes a similar feeling or image.
It really depends on the context of your writing!