To get the Repeater from Rebba, you will need to bring 'em 7 of the palette "cartridges"
These can be found in various parts of the map. If you go to Rebba at your first opportunity in the Nexus, you may have 4 or less cartridges -- you will need an upgrade or two (like the spin jump)
before you can easily secure 7 or more of these.
After finishing the "ventilation" sub-mission/area, I returned to Rebba and had 9 cartridges so far, of 14 total.(You'll eventually want all 14 to get the other upgrade from Rebba and pop the other related achievement, Rebba's BFF.)
Rebba is located at the bottom of the Nexus.
From the elevator you took down from Landing Site, you'll want to go 2 rooms east/right and 1 room south/down.Coordinates X:17, Y:16
It's the room in the bottom middle, with the low-res cat head in it