That raises the logical issue of why it has determined to never show its presence. What happens if it does reveal its existence?
Absolutely nothing. Forget what it can or can't do to the Citadel. Once the Reapers are here they control the Citadel. They can close it (and protect it), they can shut the relays down, they can kill everybody. Once the harvest has begun, the holokid could project itself to every living thing going "suck it" and they couldn't do ****. Assuming of course the Reapers don't derp out and leave the Citadel in the hands of organics and the relay network open for 90% of the war- oh wait.
This is one of the major reasons the holokid is a complete fail- it's not necessary for anything except to take pity on us and hand us its colored I win* buttons.
The Citadel- it's a trap regardless of whether the kid's on it or not. Being the crown jewel and key component of both their cycle system and their technology it's absolute lunacy for the Reapers to not have redundant means of taking control of it. For God's sake I have more control over my porn than they do over their means to completing their sole purpose in life.
The Reapers- we were just fine with mecha Cthulu being independent willful consciousnesses, we didn't need an overlord. And even if we do in order for Control to make sense, Harbinger will do just fine.
The purpose- as dumb as this synthetic-organic drivel is, it still doesn't need the holokid. Leviathans built an AI to stop AIs (lololol) it yo dawg'd them and took their form in reverence as did the Reapers that came after (meta-precedent with geth). Meet Harbinger.
But wait, what if it's supposed to be a friendly face to present us with the problem and the choices to solve it? Surely we wouldn't listen to Harbinger right? OK fine, and what's his replacement?
"I control the Reapers"

How does that help? It just said it controls the Reapers i.e everything that's wrong with our lives at the moment. Why the hell would you listen to anything that says it controls the things currently melting everything you hold dear? You could put ****** Kermit the frog in that chamber and if he said "I control the Reapers" I still wouldn't trust it. Friendly face my ass. It doesn't even have a face.
*with more devious fine print than selling your soul to Microsoft. EA