Ockham's Spoon

Arguably sexist and exploitative ...
 

Yes, that is how you would use the Trigger.
 
The END Reserve would be for just the Ultimate Power,  at least as I understand it.  If the character had more than one power that functioned like this, they might share an END Reserve or they might have independent ones.

There are a few ways you could do this, the problem is giving it a video game feel without making it a pain to track.  The simplest thing would be a custom limitation like "Power can only be used after X successful hits".  That might match what is in the video game, but now you have to track how many successful hits the character has achieved.  Also, the value of the limitation would change if the character decided to buy more levels or higher OCV so they could hit more often.  Video game powers don't always translate well into RPGs for that reason, because you don't have the computer doing the math for you.
 
If I were to build this, I think I would have the character take an END Reserve, with zero REC, and the limitation "END drops to zero if not used within 5 minutes of END Reserve charging" to make sure the Reserves always starts combat empty.  Then buy them an Aid to END, which goes into the END Reserve, and triggers every time they score a hit.  Once they have enough END in the Reserve, they can power their Ultimate Attack, but then they will have to build it back up before they can use it again.
 
 

Child: "Dad, can we go to a haunted house this Halloween?"
 
Dad: "What's wrong with the one we live in?"
 
Child: "What?!"
 
Dad: "Good night, son. Sweet dreams!"