Is the reminder text on "Red Drain" worded correctly?

Mm. Not quite. If damage is prevented, there's no damage left to cause lifegain.

How about something like:

> (Damage that would be dealt to this creature by red sources instead causes you to gain that much life.)

> (Damage dealt to this creature by red sources isn't marked on this. Instead you gain that much life.)

(I've also assumed that "its controller" is referring to this creature's controller, rather than the red source's controller? That seems to fit the flavour better, but my first couple of attempts at reading this I thought it meant the source's controller. "You" is a good word to use if you mean "this permanent's controller".)

You can probably get away with something like: Damage to this creature from red source instead causes its control to gain life

Reminder text doesn't have to be the entire comprules explanation; just enough that most people will play it correctly most of the time.

Sidenote: It's really odd to see mono-red lifegain. But eh, it's not usually gonna be used that way. A red creature with, effectively, prot-red is also not unheard of. But is pretty unusual in the modern world order.

I remembered there's an effect very similar to this already in Magic in the card Reverse Damage so I remodeled the reminder text after that.

A lot of what I'm doing with my Magic designs now has nothing to do with what might actually be done in real Magic (which I increasingly feel is headed in the wrong direction), but more of trying to make some clever parodies of how abilities might work in Magic terms. If you know about Persona or Shin Megami Tensei, you'll understand what this card is trying to do.